You can always do fittransform datasetName datasetName and it will do it “in place”. @weefuzzy warned in another thread that at the moment that may not be perfectly safe (or that might have been in reference to the buffer~ equivalent.
But in general, interface-wise, I prefer doing ‘in place’ things, as the plumbing required for having individual fluid.dataset~s and buffer~s for every step of the process is my least favorite part of programming (particularly for getting all the size things right for @blocking 2 stuff).