A little switch somewhere that let’s you lock the rest of the settings, so that when you drag-and-drop a new sample in, the only thing that changes is the sample and its loop info – this would be really swell, unobtrusive, and rather easy to implement (I should know, I’ve done so myself in a sampler I coded a long, long time ago). It could be a little padlock toggle button right next to the “transfer to new instrument” button. Easy peasy.
The benefit here is that if your customer has defined the general contours of sound – envelopes, effects, etc – but is unhappy with the way the timbre is blending in the mix, he or she can simply drag in new samples until a winner is found. (And ideally, this wouldn’t disrupt playback, so it could all be done in real time.) As it stands now, the fact that changing samples wipes the config means that, if a sound in the sampler isn’t working, the path of least resistance is probably to just give up and switch to a VSTi, which is a bummer.