Hmmm...to my ears there is a pretty big difference between an oil finish and a poly gloss finish. To my ears, oil finishes impart a sound that is, well, oily. No joke intended: the sound seems slightly smothered, and the high-end response is reduced. However, to me the difference in tone caused by replacing a poly finish from the 70's with a modern one is fairly minimal. There may be a difference in tone, but I doubt it would be noticeable in a live or recorded mix.
A lot of the conventional wisdom about the magnitude of the change in tone caused by a refin comes from the time back in the 60's and 70's when refin meant replacing a nitrocellulose lacquer originally applied in the 50's or early 60's with a more modern poly finish. I totally agree that replacing a nitro finish with a poly finish will greatly influence the tone of the instrument, but the difference caused by replacing a modern poly gloss finish with another poly gloss finish is pretty subtle.