Changing an observation charge to a full admit charge

If a patient is admitted as a level 3 OBSERVATION admit 99220, but then ends up staying past 24 hours in the hospital, and is changed to a full admission, I can modify my admit charge to a Level 3 FULL admit 99223, right?

no, you don't have to

keep the Obs level 3 and On day 2 you should bill for an admission H&P level 1 and indicate that patient now inpatient since the hospital will bill day one as Obs and day 2 as inpatient and you want them to match