Is there a time frame we must move to lightning and if there isn't a time frame what functionality will be lacking in functionality? I've heard there will be no more upgrades to the old SalesForce platform.
I haven't seen anything that explicitly states that Classic, the "old" UI, is going to be turned off. You're right that it is no longer getting upgrades. So any new features for classic are database related and not classic specific.
The answer for timeframe will depend on a few things. For example, are you a professional edition org or an org that's using primarily the standard profiles? In this circumstance you'll want to look to move by October 2019 since Salesforce is planning on auto-enabling Lightning for its customers. This will ONLY impact users on standard profiles. For Enterprise+ create or move users to a custom profile (including system admin). For PE you get 2 custom profiles. If you cannot move all your users to those 2 custom profiles, I would plan on migrating over.
