You can flash, re-frash, re,re-flash and then change everything back to how it was when you started as many times as you like!
But I really wouldn't recommend doing this just for the hell of it.
The simple answer is you can flash to any version of firmware that you have the binary for. So if you try the new(er) european version and don't like it you just need to find the binary for the original firmware and flash this to it to turn everything back how it was.
I'm pretty sure most of us would recommed trying one of the multifirmwares tho'. spivs does contain the original firmware anyway!