OK, let's ignore the database aspect and assume an individual sim.
It's possible to re-solve an individual turn using this button and rebuilding the tree from the given turn card onwards:
It's also possible to re-solve a certain turn branch by using 'Edit Tree', clicking one of the arrows corresponding to a given permutation of flop actions, editing the turn sizings for that branch, then clicking this button:
Neither of the above require re-solving the flop, and as a result are much faster than solving from scratch. Accordingly, my understanding is that given an already solved flop, it is possible to edit turn parameters and resolve all turn branches without having to redo the flop computation. Is this correct? And if so, is it possible in GTO+?