I'm not certain about read-only or input access, but I am pretty certain all e-file and REP features shut down for prior years without an active subscription to the current year software. I know people who have been frustrated that they couldn't do an amended prior return once they had left. I don't know at what level they got stuck/blocked: login, creating a copy of a file, inputting changes, REPing the return, PDFing it, and/or e-filing it.

I would make sure you have backups of the data files and PDF copies of the returns. The sales team at the customer service number would probably be the ones to give clarity on this.