I reckon you’ve got this.
To my mind, it would normally still take the same number of successes to move a hit to a different location.
However, you might want to look Defender Chooses Where He Wants to Get Hit (just about Body Targets on p 465).
As the Defender chooses where they’re hit, I’d suggest that you (playing the Troll) might choose to take the hit to the leg (as that’s what the PC’s can reach). 1 success over would then allow them to move the hit to the torso, but it would require an additional success to move it to the head.
I don’t know what your characters look like, but my PC’s tend not to have so many successes on their Strike rolls that they run out of things to do with them!
Bear in mind that you’re looking to spend your extra successes to increase your Strike damage from Incidental, to Mark, to Superb (see Add, under Melee Weapons on page 466)
Just last week, my PCs had some juicy choices to make: Should they spend their extra successes to move a hit to a less armoured position (and therefore do minimal damage) or risk the Armour roll negating their hit, but having the chance to actually injure their opponent?
You could also impose an additional Obstacle penalty for hitting the head, due to the height difference (rather than necessarily doubling the number of successes required).
I would not tend to do so, as, in a Fight! there might be opportunities and cool manoeuvres to get them there (and I love seeing my plucky PC’s try something like this - it’s costing them 2 of their successes, after all).
I think the only time when I might consider imposing an additional penalty / telling them that wasn’t possible would be if they were attacking a dragon’s tail and wanting to hit its eye, for instance.
You might, also, consider what they’re doing against Task and Intent (pages 24 - 25).
Is what you / they are trying to achieve possible?
You might, for instance, rule that hitting this giraffe in the head from ground level with your fist is not a suitable Task (whereas shooting it with a bow would be).