In the case of the FR you have it is likely the results would be wonderful. WIth the non "F" version it wouldn't be terrible to change the cantilever since it isn't the preferred (by collectors) "F"
Re-tippers offer different stylus/cantilever combinations. Among the materials offered for cantilevers is aluminum — that's what the
F uses — and the stylus is a line-contact. So I think a full replacement would not alter the sound characteristics very much — aluminum + line-contact is also usually the least expensive replacement option.
Changing the cantilever to, say sapphire or ruby,
would change the sound. It might be a great change, and I'd love to hear it. But if I were retipping my
F, I wouldn't take the chance, I'd stick to the original — which fortunately is also the easiest.
The Fidelity Research FR-1 MK 3 is a low output MC and tends to like higher mass tonearms with the ability to do it full justice. And it's a very excellent performer, a Japanese LOMC classic.
This may be a bit nit-pickish, but I think Kent misspoke — it's a MC's low
compliance which makes it a better match to higher mass arms, not its low output. But Kent knows a lot more than I do, and will correct my correction if correction is needed.
To his broader point, I've used the FR1-Mk3F in higher-mass arms (SME 309, Linn Ittok, Syrinx PU3) and he's right, it's great match (the Syrinx was my favorite, btw). But I also used it in a SME 3009 II Improved, considered a low mass arm, and it was still stellar. Moreover, it was the fixed headshell version of the SME, the lowest-mass version. (Because of the FR1's weight (10g), however, I augmented the counterweight, and this did change the arm's effective mass, though I can't say how much.)
As I wrote above, the Mk3F is not a "fussy" cartridge. My point is, if you have the chance to get one, don't hesitate — you don't have to worry if your current tonearm or preamp can "handle" it. (Unless you're using an ultra-low-mass arm like a Mayware, I've never tried it so can't comment.) Moreover, if you seriously upgrade your phono system, the Mk3F will just sound even better. It has no weaknesses.