6l80 transmission shifting issues

[Rich G]

Recently swapped a 2016 L83 5.3 with a 2018 6l80 trans with 8000 miles on it in a 84 Pontiac Grand Prix for my brother. The engine and trans were not a paired unit I had the wire harness and programming for the ECM and TCM done by Swap Time. The problem we are having is the trans will shift through the gears but acts like it is slipping a little then shifts through gears. We have not driven the car for fear of messing up the transmission. Could this be a programming issue or some type of relearn to correct this issue. Any advice would be appreciated.

[Snowcatter]

Following...I believe I am having the same issues you are. Mine shifts firm when I'm on the throttle, but acts like it slips when accelerating slowly. Like between 2&3 or 3&4 the rpms go up briefly before engaging the gear. Mine is a 2010 L99 and 6L80E