One thing I learned long ago, always have a file that isn't dependent on a particular software. Any software, particularly along the lines of Ai and DRAW, can take anything out and make compatibility a bear. And it doesn't matter if we like it or not, that's an ability that they have. Or if you no longer use said software (they went under or just don't use them any more whatever the case may be). Especially if we are talking about decades of files. I started doing what I do back in June of '94, I still have those files that I first did. While the production files are still good to go, the master files are not, even though I still use the same software that I did way back then.
I'm not saying to not have a master working file, but don't have that as your only source of a vector backup (if dealing with vectors). That may mean having a second file that details what was used if there needs to be any changes made etc.
Although, I will see as I've gotten older, the less I like vendor lock in.
That forces one to go back, re-open all those old files on an old computer running something like CorelDRAW 9, and save the files forward.
That actually depends. Not always needing to be the case, but it does depend on the situation. I personally don't like keeping old computers around, so I'll use software solutions to keep legacy software going or, I'll use a different software period that's current. Typically those older files will open up nicely on newer 3rd party software. It's when trying to open recent versions of the same file format in those 3rd party programs that can be a booger.