I’m biased towards doing things this way and think it’ll probably be best in your case too.
One way to approach this is to consider there being different milestones on your way to a complete launch, that might look something like this:
- site made available to close collaborators
- site made available to beta testers
- site made available to pilot group
- site made available to whole existing community
- site made available to new members
- end of support date announced for certain activities in old community
- certain activities no longer supported in old community
At each of these stages, you slowly expand who is included, iterate on the site until you believe it’s ready to move on to the next stage. Do things required for that next stage, but not the stage beyond. Learn what’s really needed as you go.
The last two steps could also be repeated in a loop, as you replace activities one or a few at a time. Maybe eventually you spin down the old community entirely. Or maybe it remains indefinitely to support a smaller set of activities.