If a flip summon (via attack) is negated, can I change attack target?
I attack a face down monster (1900 att to 2500 def) and when the defence monster is flipped face-up I activate Forced Back (a counter trap which negates the summon and returns it to the hand) does that mean I still take 600 damage, or do I get to attack again, directly this time


What you did there was not a legal move.

The monster was flipped face up by your attack... It was not flip summoned. Those are different things (the flip summon is an action done by the monster owner, flipping face up is a result of your attack and is not a summon).

From the wiki we have this, which clarifies the difference:


  A face-down monster flipped into face-up Attack Position by a card effect such as "Book of Taiyou" or "Ancient Forest" is not considered a Flip Summon.


So, technically you couldn't have activated that trap card, and should have taken the damage.

You are correct that when the number of monsters changes during an attack there is a replay, but this was not the case.