I don't think there is any regulation about switching pellets in FT, as long as your gun is under 20 FPE in any configuration you shoot it....
There are two primary reasons that heavy (ie over 10 gr.) .177 pellets are popular in FT....
1. When you are near the 20 FPE limit, lighter pellets will be travelling too fast.... in the transonic (or supersonic) region.... and therefore have worse accuracy (and ballistics)....
2. Heavy round-nose pellets will usually have a better Ballistics Coefficient (BC) than lighter ones.... giving a flatter trajectory....
People shooting in the 12 FPE (International) Class will typically use medium weight pellets such as the JSB Exact 8.4 gr.... or even standard weight pellets (7.9 gr).... because you have the opposite problem.... you need the velocity of the lighter pellets to get a flatter trajectory.... The same applies if you are shooting a .22 cal in FT.... You will probably use a standard weight pellet to get the flatter trajectory....
For reference:
20 FPE ~ 920 fps with a 10.6 gr. pellet.... or 790 fps with a 14.3 gr. (.22) pellet
12 FPE ~ 800 fps with an 8.4 gr. pellet.... or 825 fps with a 7.9 gr. pellet
Whatever setup you are using, try and stick with true round-nose pellets (JSB Exact, Crosman Premier, H&N Baracuda/Beeman Kodiak) or similar to get the best possible BC.... over 0.030 is preferred....
Bob