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.
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Bob