I often keep just the most previous version, then delete older versions. I want to make sure the new version works properly and handles everything i threw at the previous version. Usually after a week or two i never open the previous version again. So, to answer your other question ... the next time i open an old project i'll open it in the new version.
Keep in mind that if you save your project in the new version it will no longer open in the older version. If that's an issue then you should copy the .vf file and work on the copy in the new version, keeping the original .vf file aside in case you do want to use the previous version.
Depends on the program and how much i like it. I tend to stay current with the whole SONY Media Software line. A big part of my reason is to encourage the developers to keep working.
My last Microsoft Office upgrade was to version '97, and i'm completely happy with it. No newer version has offered even one single bit of anything beyond what '97 had that interests me in the slightest. In fact, i've used newer versions on other people's computers and found them less useful. So, i remain 11 years and 4 versions out of date on that one.
I upgrade OpenOffice.org as soon as it gets out... 'cause it's free.
I upgraded from VMS6 to VMS8Plat. I'll skip 9 as sony decided to stick DVDA 4.5 and I don't really need the new version 9 features.
Ivan, I would suggest you move to v9 if you buy an HD camera hat has better compatibility with, or if you want to be export in h.264 manually. Except these reasons, yeah, stay with v8.