If the existing work looks good and
I could easily isolate the grounded conductors from the grounding conductors I would simply leave it in place and feed it with a 100 amp breaker.
You did say
a 100 amp panel but it has reached its load ampacity
Does this mean it has tripped the main?
If yes I would still leave it in place but move one or two of the largest circuits over to the new panel. A range, dryer, AC unit, etc.
Hard to really
say what I would do without having seen everything.