What rules changes have there been in Uno?
A definitive answer would require someone who has a collection of all the printing of UNO since the beginning in 1971 to present day. The only one likely to have such an museum quality collection is the estate of the inventor of the game, Merle Robbins, an Ohio barber. He spent $8,000 to have 5,000 copies made. He sold the rights to the game to International Games Inc., which was bought by Mattel in 1992, who now owns the rights to the game.
Here is my best attempt at answering. I am going to assume that the 1983 International Games, Inc. edition is the same as original 1971 printing. The differences in the many spin off versions of UNO ([30th Anniversary Edition], Batman UNO, Hanna Montana UNO, Spongebob UNO, Dora UNO, Mickey Mouse Clubhouse UNO, Cars 2 UNO, Harry Potter UNO, etc.) are thankfully not included. I will mention that in general, the spin-offs have nearly identical rules, but instead of including blank cards like the most recent printings, these blank cards are replaced with special cards that have some unique effect. I would guess that the reason for the +4 blank cards from the original printing is related to card manufactures changing from 54 cards=9x6, to 56 cards=8x7 sheets.
The complete rules for the 1983 version are listed, with only differences for other versions listed. The 4 previous versions (1971-1981) are assumed to exist, because they are listed as additional copyright dates within the 1983 rule book. There are gigantic gaps in the editions, because I have been unable to find scans of the intervening years. To summarize the differences though, most versions have nearly identical rules. The earliest version only uses the (0-9) face cards for First Dealer selection, requires a challenged player to only reveal the color of the cards in their hand, if &quot;caught&quot; in a challenge they keep their Wild Draw4 card, and specifies a penalty for suggesting plays to your opponents. The 2001 editions (including 30th Anniversary) have a printing error in my mind, because no other versions I have seen make you Draw 4 cards when failing to call &quot;UNO.&quot; I don't think the game rules have changed much in 40 years.

1971 First Edition
1973
1979
1981
1983 International Games, Inc.


Components - 108 cards, 4x Wild, 4x Wild Draw4, 4 colors each;19x (0-9);2x Draw2;2x Skip;2x Reverse
Object - Score 500 points by emptying your hand before your opponents
First Dealer - Non-number cards are not used. Players who draw a non-number replace it in the deck and draw another. (This differs from all subsequent versions I have found. In newer editions, non-number cards are not replaced in the deck allowing that player to redraw a card. There are other differences from later editions that will be noted, but this difference will only be noted here). Shuffle and deal 7 cards to each player, place the top card in the discard, and the remaining cards become the drawpile. The person to the left of the deal starts. With the exception of the Wild Draw4 (replace in deck redraw) and the Wild (starting player decides color), Special Word cards affect the stating player in the normal manner.
Summary of Play - Play a matching number, color, word, or wild to the top card of the discard. If a player cannot play, they must draw a card (which can be played immediately if it matches). Play passes to the next player. If the drawpile is depleted, resuffle all cards except the top card of the discard to form a new drawpile, and continue play.
Reneging - You may choose to draw a card, even if you have a playable card in hand. If you do so though, only the drawn card can be played. You can play a drawn Wild Draw4 card, but might be subject to getting &quot;caught&quot; if you have a playable color card in your hand.
Special Word Cards - Wild can always be played. Choose a new color. Wild Draw4 can only be played if you do not have a matching color (matching number/word is ok) without being subject to &quot;caught&quot; rules. Choose a color, the next player draws 4, play continues with the player after the next. Draw2 the next player draws 2 cards, play continues with the player after them. Skip the next player is skipped, play continues with the player after them. Reverse the direction of play is reversed until the end of the hand, or the next Reverse card.
Going Out - As a player plays his next to last card, they must say &quot;UNO&quot; before the card hits the discard. If they fail to do so before the next player has draws/plays a card and are &quot;caught&quot;, they immediately draw 2 cards. After the next player plays/draws, they are safe. When a player plays their last card, the hand is over. Points are scored, all cards are collected, the deck is shuffled, and a new hand begins with the player to the left of the dealer. If the last card played is a Draw2 or Wild Draw4, those cards are added to the next players hand before points are scored for the hand.
Scoring - When a player goes out, all other players card values are added that players total. First player to 500 points wins. Wild, Wild Draw4: 50 pts, Draw2, Skip, Reverse: 20 pts, All number cards (0-9): Face Value. Variation: Running totals for each player can be kept instead. When one player's total reaches 500 (or some other value), the game is over and the player with the least points wins.
Penalties - Fail to call UNO on next to last card draw 2 cards, Make a suggestion to another player on what card to play draw 4 cards, Play a Wild Draw4 card with/without another legal color draw 4 cards. The next player is the only one who can challenge. If they do, you must reveal the color of the cards in your hand to verify if the play was legal. If it was, the challenger draws an additional 2 cards (6 total), if instead you are &quot;caught&quot;, you draw 4 cards. (Challenge In this version, only the color of the cards in your hand need to be revealed. Subsequent versions do not clarify what &quot;reveal your hand&quot; means. Another peculiarity of note here, is that in this version the Wild Draw4 card is returned to the &quot;caught&quot; player's hand, and the a correct color card is played. All subsequent versions leave the Wild Draw4 in the discard pile. This does have an unusual effect on the gameplay. With this rule, it is not possible to sacrifice getting &quot;caught&quot;, with the only goal of changing the current color.)

Note: Houserules you might have played with. Piling On/Progressive UNO - It allows the normal recipient of a Special Word card to pile on the result to the next player. (i.e. If someone plays a Draw2 card on you, you can play a Draw2 card, making the next player draw a total of 4 cards.). Draw until you have a playable card - Instead of drawing a single card when you cannot make a legal play, you continue to draw until you have a playable card.

1984-1992 International Games, Inc.
1992-2001 Mattel, Inc.
2001 First Dealer is highest point value card. Going Out failing to call &quot;UNO&quot; and getting &quot;caught&quot; draws 4 cards, no penalty for opponent play suggestions. 4x Blank Cards for adding special rules, or just replacement cards.
2002
2003 First Dealer highest number goes first (word cards are 0). Penalties New penalty for making incorrect plays, the offending player takes the card back and draws 2 cards.
2004-2006
2007 Deluxe First Dealer highest number goes first (word cards are 0). Penalties no penalty for opponent play suggestions.
2008 First Dealer highest number goes first (word cards are 0). Penalties no penalty for opponent play suggestions. First mention of Houserule Progressive UNO.
2009-present