I've got some ideas I want to look at for this situation, deleting existing subscriptions (and thus removing records of purchases) isn't one of them though. If they're upgrading to a higher tier subscription, they could renew the one they have now and then upgrade it. I know it isn't ideal, but it's a working upgrade path.
However, not seeing an error message when the member tries to log in is not intended, I have filed a bug report for that and we'll take a look. They should be seeing an explanation for the issue instead.