If you use it in GMRS mode... yes it will adjust it's power output for the GMRS channels you are on. It will do 5 w for 1-7, 0.5w for 8-14, and it's maximum output on 15-22 and the repeater channels.
Wondering why you want to go to an H8. Yeah it can put out a few more watts on the higher channels, I just find that it's programming and other options seem to be less than what the H3 can do. I guess it is a nicer larger size and larger keypad buttons... and it has some water and dust protection ratings over the H3.
TrikeRadio's post in 10w TD-H8 for GMRS use was marked as the answer