Probably they will do it like the GT Sport. There was a prompt to import liveries from GT Sport to GT7 when it was still alive. The livery makers should import the liveries to the next game. IIRC I imported my own Suit and Helmet liveries from GT Sport to GT7. Car liveries worked the same way too. Or maybe even PD will automatically convert the liveries from GT7 to next game. When they release the next game, GT7 server should be still up for a few years. And after the GT7 server is shut down (which is probably few years after next game which we don't know when it will be released, probably with PS6, so it's probably 2030), the cars from our garage will be converted to their default colors with no way to apply custom livery (like the GT Sport right now).