Impact of selecting your level 30 class change

[Sateist]

I’m guessing that you are talking about losing access to multiple role actions (e.g., Protect, Break, Drain, etc.)? .

That is correct! Playing Conjurer, I felt that I lost a lot of good useful role actions, where as for Archer, I didn't mind losing them.
This struck me as a quite big difference in class abilities, at the moment you switch to your new job at 30.