Mechanist can't be swapped out later.
Now that I'm looking at it on my PC and not my phone... I have now noticed that it does in fact say "This Civic provides no additional benefits and cannot be removed once picked." for both Syncretic Evolution and Mechanist. Darn phone making it hard to see that. Thanks for letting me know earlier, would have been at least 10 hours before I knew for sure.
Still think both are going to be great and fun civic options! Especially considering the early game advantage both can give someone.