The 5.9 will run on a 5.2 comp, just not to its full potential. Flashing is usually not done in favor of just picking up a mp360 comp that will add more pick me up and support some mild mods (cams/heads/etc/littlestuff)...
5.2/5.9 swaps are straight foreward plug and play other than some balancing/flxplt work, i dont see how you could mess it up unless faulty hardware on the donor or you hooked up the sensors wrong.

What year rango and donor?

 

I wouldn't have thought they would run on the same PCM?

 

5.2 to 5.9 eng will work, but its not feeding the full potential to the eng. as the settings are derived for the lesser 5.2. While it does counter act the swap, it can be used until the purchase of a 5.9 set.

 

Just learned something new......Thanks!