Can a letter of recommendation be rescinded after submission/acceptance?
Anything is possible, of course, but (warning - opinion coming), I think it would be very unlikely. First, I doubt that your mentor would do that, though I don't know the person so can't be definitive. I would not, anyway. Second, if you have been accepted (and told them you are coming) you have at least an informal contract that is unlikely to be broken. In some places it would likely be against regulations to withdraw an offer after acceptance. But again, I suppose it is possible that it would happen.

However, you might be questioned by your mentor or others about this, so be prepared to give an honest reply. People learn as they live and they change their minds about things. Other people generally understand that and accept it as long as it doesn't cause disruption. 

But I think your worry is probably based on very little. (Sorry about all the "weasel words" here.)