… without having to retune/transpose each sample, or modify pattern data?
You can right-click on the instrument’s sample list, choose Select All and then operate on all the samples at the same time.
NM, found this, works very well – New Tool: Transpose Instruments
/* EDIT */ oops, got postjumped 
Yeah, that works when all the samples are at the same frequency. If they have different values, they all get assigned the new one.
The tool above does the job of relative-tuning each sample one by one, works great.