So the Sole win condition of this game comes through a mechanic called  fame, it's basically victory points.  If you stay with your current culture rather than changing, you get a bonus to all fame gains.

When you change culture you lose the ability to build it's special building, along with it's affinity (I.E if you are Babylon your affinity is science, this means you get more fame from gaining technology.) 

Also just to clarify however you change era keep or change culture, you get access to all the new technology associated with the next era.

While Optimal strategies and the meta of this game are yet to be defined, (some nations such as the Huns are still getting significantly reworked, according to comments from the devs.) I get the strong impression, some cultures  especially those with really strong buildings will actually be better off transcending than switching, depending on circumstances of course.

for instance say you have a start where you don't have to invest in warfare, you have plenty of land for new cities, and either you are surrounded by mountains, or you have friendly peaceful neighbors, if this is the case then getting a powerful new unit is not going to effect your game at all.  In this scenario, if you change culture what you are essentially doing is, changing affinity, unique building, and gaining a new trait, (I'm trying to remember what they are called.  It's basically a bonus tied to your culture that lasts the entire game, the Zhou one makes districts cost less stability.) at the cost of less fame in the next era.  If you are Babylon and your empire is set up to gain tons of science, then it actually does not make sense for you to switch to a different culture, from a min/max  I want to win the game standpoint.