Upgrade from what, 5/6/7 speed?

The measurement you are giving have nothing to do with any speed upgrade, what your interested in is the OLD of the hub, if it's freewheel or freehub, how many speed your rear dérailleur is, how many speed your chain, cassette, right hand shifter are.

If you have a caliper brake on an MTB, this would indicate that its either from from the 1980's, which will give lots of issue with nothing matching current standards, it's a 1990's ultra low end BSO, which has been re-badged as a Hardrock, or they are not calipers, and you need to ID what type of brake you actually have.

For the rim, you won't notice the difference in 25mm-28mm other than the 28mm will probably weigh more.