I think this is a smart thing to do and I don’t really see any contradictions in this with any other concept/mechanic. We would be reinforcing the strategy of migration when environmental tolerances start to become a factor, so it would make sense to give the player more unique and defined tools to manipulate their population.
In the original concept as MirrorMonkey said, it seems that the player would have some sort of credit/respawn point system which grants them the ability to switch species after death. The more closely related a species is, the less points it would cost to play as them. So it could cost a point to play as your closely related ancestor and several to play a rather distantly related organism. The player could then earn more points through an action, perhaps by surviving a certain number of generations.
Though of course, a unique game mode where there is no lose condition is very possible, and would be a strong reinforcer to the idea that there isn’t necessarily an absolute win in evolutionary-terms. It could also be a nice way for newer players to experiment with Thrive.
But again, that discussion is a decent bit ahead of us for now.