If there's nothing wrong with the 3 x 10, and you don't mind it over the 9 speed, then yeah of course I'd do that (and assuming it's compatible). Worst case you wasted an hour putting it on and need to replace it. Best case you save yourself a few hundred dollars that you can allocate elsewhere.I recently had my 2013 Trek Superfly mtb converted from a 3x10 to a 1x 11. I still have the old drivetrain and it is in good shape. It is a mixture of Shimano Deore, SLX, and XT parts. The wasn’t a lot of miles on the 10 speed chain either.
I have an 2010 Cannondale Trail 2 with a 3 x 9 Shimano Acera/Devore that I use for gravel/paved surfaces. The Cannondale’s chain and cassette are due for replacement from normal wear. Is is worth swapping the entire drive train out with the better spec 3 x 10 rather than getting a new 9 speed cassette and chain?