Change Gnar with 'R' if...
...he has 100% rage.
Everything else stays the same...
Simple as that. 'R' will change Gnar to Mega-Gnar if he has 100% rage. He still glows red indicating he can change, Lvl6 he gets the point so he gets the fun little passive bonus as Gnar and the FU toss as Mega-Gnar.
tl;dr: Change Gnar with 'R' if he has 100% rage.