That goes for any transmission. If you have a bunch of miles on a trans and you goand increase pressures and such, yeah, you can have problems. More than likely what happend with you was your seals were worn around the second gear clutch piston and the increased pressures made theleake worse and you lost enough pressure on the clutch pack to fry them. If you raise transmission pressure you ave to know that the seals in the trans are in good shape in order for the trans to operate properly. "Shift kits" and upgraded valve bodies (which have shift kits plus other proprietary stuff) all increase pressure in a number of dofferent areas including line pressure, lock up clutch pressure, clutch engagement pressures, and the speed at which these pressures are generated.
If you have a brand new truck and have 10k on it, your fine. Anything more than about 30K, depending on how the trans has been treated, rebuild. Do it right the first time. That or wait until you can to play.