If they are not dead, leave their accounts as is. though if you feel like you want to do "something," add them to another group, and adjust the permissions as you choose. This makes adjusting the permissions on the members of that new group easier, if you choose to updated them later in time.
Some people who dont use their bikes any more may come back to sell a bike related item later, or look for an old friend or pm, or just come back to see how things have changed from year to year.
Some may never come back.
Though, whatever you choose to do, and if its important to you to do something, make an announcement to your current members, add a clause in to the new user registration/rules page, and then do what you want to do with those members. This is 100% your call. Its your business that you are running
As a side note/some backstory slightly related to this topic; because I was working on other projects at the time (but still running and customizing ubbt software for my personal businesses), I didn't have any need for interacting on the ubbcentral.com forums from 2006 to 2011. Though my account here (and ubbdev.com) was created in 2001 (?) and i was actively reading until 2005, but I had not posted much until around 2013. And here i am now, as a person currently dumping the most amount of ideas => code in to this software. If I had came back in 2013 and my account was gone, i probably would also be gone... somewhere else. working on some other enjoyable coding projects. This is just something to think about before making any major account changes. Whatever choices you make now, may have good/bad affects down the road.