While I'm definitely curious about whether switching religion or culture will be possible, but I actually would be perfectly fine if it wasn't and I think in the case of the Daidochi it would probably be bad from a gameplay standpoint because it would be a way of instantly overcoming their primary weakness (being foreign conquerors whose population will resent them). What reason would there be to remain Greek as Ptolemeic egypt, for example, if you can just culture flip and instantly remove all aggressive expansion unrest penalties from the population?
The Daidochi are going to be crazy strong at the start of the game. Forcing them to spend time and energy Hellenising the regions they control or else have to manage AE carefully seems like it will be a very important part of stopping them just blobbing out and absorbing all their neighbours with impunity. Plus, it would be cool to see rebellious natives rise up in these regions and establish their own states, rather than just having the Selucids just become the new Persian empire in every game.