I want to file an opt-out but the proprietor has changed and the European Patent Register can no longer be updated. Can I still do it?

Yes. While you will not be able to request a change of proprietor given that entries in the European Patent Register cannot be changed outside the periods defined in Rule 85 EPC, this will have no effect on requesting an opt-out. To request an opt-out, all the requestor has to do is declare that they are the rightful owner of the European patent (see item 4.6 of the UPC FAQ on Opt-out). See also Rule 5(1)(a) RoP UPC.