Their IMAP implementation is terribly broken. It will most likely just create brand new messages and not delete the old ones. It's one of the many reason I moved off gmail (to self hosting) in 2012. Another being domestic spying .. which everyone has forgotten about apparently.
Which doesn't actually matter if you're in the 96% of people that are not from the USA.
This argument "but we don't spy on USA citizens so now we're all good and ethical" doesn't make much sense to me. If a Dutch company allegedly spies on foreign customers, it really wouldn't change my (Dutch) opinion about my privacy at that company.