[Gmecm] 200 3800 OBD2 question

[Mike V]

I installed a used 3800 in my 2000 Impala over the
weekend and it started & idled smoothly.
Compression was good in all cylinders and it looked great.
During initial warm-up & fluid check, it started to run very rough.
A test drive showed it to be way down on power.
Is there a re-learn procedure I need to initiate with a scan tool?
I will borrow the appropriate scan tool today.
tia
Mike V