I have seen this happen (not to ones I owned, but ones that I had seen were going to drop and had planned to snap up), but it was when they dropped. I don't think, surely, they can do this to domains that are registered? Certainly hope they can't!
Be careful ..
Registries most certainly CAN change whatever they want including pricing WHILE you "own" a domain. They do now have some restrictions in giving you X number of months notice though. (Also remember that nobody ever actually legally owns a domain either)
That being said .. most registries use common sense and don't jack up a regular domain to a premium domain while it is "owned" by someone as it would bring a horrible backlash from the community an a ton of negative press within the domain community.
Several years ago (before I got more seriously into domaining) I registered a couple dozen incredible .news domains. Names like USA, Diet, Health, App, Movie, Mortgage, Canada, Music, Job, Fashion, Sports, World, Insurance, Tech, NewYork, Vegas, Miami, Advertising, etc. I was super excited .. I started making development plans and setting up a partnership for a couple of them .. then a couple of days later GoDaddy reversed the transaction. I realise it wasn't their fault, but it would have been nice if they would have stood up to the registry and let me keep one at least as compensation for me essentially losing a few days of my life! Still pisses me off to this day thinking about it!
But yeah .. I don't actually think it's unreasonable for them to change the price or status after a domain has been dropped.