Make an Epic Project Cancelable

At least work it up to bronze.  I don't know what would happen if you didn't....  Probably just leave it as a non-epic structure.

There is no way to cancel it.  Once you start it, it will go to completion or 24 hours, whichever is sooner....

I wonder if you didn't make it to epic at all if it would retain it's helmet.  Probably not.  I'm just thinking out loud here.  If you care to test it, feel free to leave it be for 24 hours and report back what happened.

If you don't get to bronze within 24 hours, the building will revert to normal and the 12 hour epic building cool down will start again.  So, wait it out like SomeSimGuy suggested if you only want llama or cheetah buildings.