The terms of membership - indeed the fundamental principles of what constitutes the club - will always be open to renegotiation. There are areas most notably on trade, federalisation and funding of social projects where we can find common ground with other influential members within the EU, most notably Germany.
That's not something that could be done in the old Soviet Russia, so I think the comparison from the OP facile.
I know there are many people in the UK who dislike the EU project enormously, referring to it,half-jokingly one imagines, as the "EUSSR", but really - the likelihood of another war in Europe, especially over the membership or the terms of the EU agreement strikes me as extremely improbable.