Langjan

if all the existing panels are 48V, thus 3 X 48V = 144V per string and the new higher power panels are also 48V each and thus will provide loads more current, but also at 144V for the 3 panel string, then all should be ok, as long as your Growatt can handle the current, this should be fine without too much trouble, you presumably would have to wire in the new panels with new cabling, and not parallel them up on the roof or wherever the panels are located, since presumably the existing cabling isn't sized to carry the 4-odd kW's worth of current, but if it is, then even this won't need worrying about...