Probably not. Diaging a trans in one day with no parts no dtc is a bit hard.

The stall test will tell you what gear is slipping but will not fix any thing.

Lets say 2nd gear fails the stall test, you would then check clutch pressure(you would need the fitting for the pressure port and a pressure gauge). Lets say their is proper pressure the clutch pack would be bad=a new trans or a rebuild. Lets say their is no pressure you would have to diag the shift solenoid and the pressure control solenoid or even the valve body.

The fluid change might help so in one day you could do that and see what happens.

Just don't be surprised when the dealer charges you 90 bucks and tells you everything is ok or you need a new trans or a 120 dollor trans flush.When probably all you need is a shift solenoid B or A. I would almost try to find a trans specialty shop that rebuilds trans over the dealer.