Sage Instant restore misery

I moved my small practice away from Sage some years ago to VT, but maintained an Instant program in order to support a single client.

Over recent times this has worked well, but the client upgraded without me knowing and so I had Sage meltdown when I tried to restore their v 18 data. I have uninstalled and reinstalled my original v.17 program 3 times and each time I get the error message, 'this program was last used with v.18 data do you wish to upgrade now'. It doesn't matter if I click 'yes' or 'no' the program remains unusable.

Is this a Sage conspiracy to force the purchase of an upgrade or is there a simple way round this problem?

Thanks in advance.

PS. Just to save time this isn't a question about whether or not you can restore later version data to earlier version programs, I know you can't. It's about whether as a result of unwittingly attempting to do so, the original program is rendered unusable without purchasing an upgrade.