Back to the original issue, I see only 2 options:
1-find someone with an older setup, open the sessions and take notes or screen shots of the settings you need.
2-don't be afraid to just start over with a different but similar plugin.
Some plugins did not make the transition intact(why those plugins won't come up on newer systems). If you consider how often before DAW's, when engineers had to resurrect a complete mix with next to nothing to go on, we are still miles ahead on recalling a mix. Personally, the few times this issue hit here(and it has), I simply take it as a challenge to better the old mix(and I usually do). That might happen due to newer and better plugin options, or it might be down to fresh ears
If you mention the plugins you are wishing for, I bet the folks here will have some good suggestions for substitutes
