Opening older versions of Garageband files in latest version of Garageband

But this is the case for every DAW, it's not unique to GarageBand (or Logic Pro, for that matter). Newer computers, newer software always requires updating older files. As far as I know, other than perhaps wax cylinders or vinyl records there are few truly enduring mediums for recordings. Everything created digitally will continue to be in a state of flux as improvements continue to be made over time... and we'll have to keep opening and saving updated versions of our files every single time.

Did you keep Time Machine backups and do your backups extend far back enough that you'd be able to resurrect these old loops?

I know in Logic Pro it's far easier to re-download older files. That's a feature Apple should consider bringing to GarageBand... but that won't help you unless you have an old backup or a way to reinstall loops from another source.

Yes, the same has happened to me. I will do a full search for older loops and sometimes and they come up missing. In my case, I usually end up replacing them with something else.

In my recent experience, GarageBand has become increasingly unreliable and I'm now trying to transition away from using it. Regrettably, the more complex Logic Pro is the only serious option at this point if one wants to retain prior work done in GarageBand (in my case, many, many years of work). Apple should really comb over these posts and work directly with people posting serious problems.