I'm considering upgrading my GF Tarpon to a 27.5" wheelset. Is this possible? And how will affect the ride? In general, I've heard people say no, and some people say yes (nothing specific about this bike, though).
Why do you want to do this?
The problem with switching wheel sizes is normally the brake mounting for rim brakes, and the tire clearances. I have a feeling that you won't be able to line up the rim brakes when you do this (if you had disc brakes, this would be simpler - if the rotor fits, it works). The following thread here says that Velo Orange's Grand Cru canti has enough adjustment for swapping 26" wheels to 650b wheels on some bikes (probably, but would require switching to short pull brake levers) and Paul's MotoBMX brakes may work as well. Your current brakes may or may not have enough adjustment to do this. This is all assuming of course you can even get the 650b wheel in with your desired tire. You could run a staggered setup (650b front, 26" rear) with a new fork at the worst, but this is expensive and likely would require you to adjust things due to the height difference.
A switch from 26" to 650b will move your bottom bracket up 1/2 an inch.
I think this conversion would be largely pointless, especially on a cheap bike like a Gary Fisher Tarpon. You're better off picking a better tire for your purposes unless you REALLY need the extra half inch of bottom bracket height (in which case, you should probably get a different bike). You'd have to buy new wheels and possibly new brakes, which puts you a good way towards just selling the bike and buying a 27.5" er to begin with.
On the other hand, Sheldon Brown points out that the 700c to 650b conversion can be useful.
I don't believe this is possible for a number of reasons even if the back wheel fits:
For the cost of a new fork as wheel set you'd likely be able to find a comparably spec'd 650B.