Basically, we tagged the circuits as they were in the old panel, ID'd the neutrals, &/or ID'd the 240 volt and anything else that was 'strange'. Then we terminated on proper amperage CB's & energized one at a time.
When a problem surfaced...we did what was necessary to correct the issue. Occasionally that turned into a bit of additional T&M work.
In this case, experience with 'old' work is the best teacher.