My hybrid (Gaint Roam 2) is currently fitted w/ the stock low-end Shimano FC-M371 3x9-speed crankset...which has riveted gears.

I'd like to swap out to something w/ replaceable chainrings, but I can't seem to find much definitive information on an easy swap. It would be nice to reuse everything else...

Assuming that modern Shimano square taper cranks all fit the same JIS pattern....how do I find the proper "offset" to keep the chainrings in the same relative lateral place?

Do I have to swap out the BB too....or is there a direct replacement that will work with the current BB/FD/etc?

I'm capable of doing the wrenching & small adjustment tweaks to the FD if needed, but yeesh are there a LOT of options out there!

Thanks for any help!

You can usually get a BB spindle length recommendation when you buy the crankset. My bet is that, since you are upgrading the crank, the new BB spindle length is going to be significantly shorter than what you have now.

Some years ago I upgraded the crankset on a Trek hybrid with a Sugino 600, a real workhorse that's still on one of my bikes. I don't recall switching out the BB at the time, but I wouldn't be surpised if you had to. Sugino XD600 Triple Cranksets > Components > Drivetrain > Cranksets | Jenson USA

Agreed that swapping a crankset sometimes also means the BB is in play too. I always mention this possibility and include it in estimates. Some of the time where the OEM Bb would dimensionally work we find that it's condition is bad enough to warrant replacement. I usually return the old BB to the cust anyway. Andy.

Since your bike has a 135mm rear hub, I'd go with a low end Shimano mountain bike crankset with external bottom bracket. You won't have any chainline issues and external bb are very easy to service. If you go with a square taper bb and crankset there's always the chance the chainline isn't right, take care to choose the two correctly.

there's quite a few options on ebay, usually out-of-box or takeoffs can be had for cheap

Thanks everybody!