Being able to downgrade is a major distinction between the aliens and the humans. Humans often change or move their base as the aliens evolve to make it more resistant to the higher alien classes. I think that allowing alien downgrades would make the aliens overpowered.
Really, it's up to the alien players to choose an appropriate class depending on the situation. E.g. Choosing a tyrant when the humans have built in a tyrant-proof high area (like the pipe junction in Arachnid) probably isn't the best idea. If you're already a tyrant then well... Poor you. Sorry if I sound elitist here, but I like it the way it is now - it'd be too easy if aliens could de-evolve. In these cases I just die attacking and respawn.
