I was thinking of changing my MTB front rim from 24" to 26" and to a wider 2.125 tire. What changes can i expect?

From what I understand, you'd have to change the fork as well. Plus, it will most likely raise the handlebars and make the bike more "slack". Perhaps someone more experienced can jump in now.

what bike do you have now?

If you run a rim brake, you can expect the brake to no longer reach the rim. Have you checked to see if there's enough clearance to make that change?

its a old raleigh bike and its disc braked and theres no issue with clearance. Wanted a front larger wheel as i thought it would roll over pot holes a little better.