In Settlers of Catan, is there a transition/evolution between "building" and "development" phases of the game?
It depends

In general, the first few rounds are focused on expanding to a new settlement or two. You start the game with two free roads, but to build a new settlement you need to be a minimum of a hex vertices away. If don't expand your road and build a settlement, these free roads are wasted. It isn't unusual to build roads towards the Harbors so you have favorable trades.

It is possible to win the game by not expanding at all though, although this is rare. With good access to Rock, Grain, and Wool you can buy Development cards and build Cities. There are 5 yellow Victory Point cards and 2 bonus points for the Largest army. Those combined with your starting two Settlements, one of which needs to be upgraded to a City is sufficient to win the game. The reverse is not true, since you can only receive 5 Settlements and a bonus 2 for the Longest Road, you would be 3 points short of winning.