Funny you should ask. We just did a bunch of stuff on precisely that issue, i.e., the need to provide consideration for contract changes. If it’s not up yet and u want to see it, email me at glennd@bongarde.com and I’ll send it to u.
As for your specific question, NO, a raise that was previously agreed to would not constitute consideration. Basic contract law: Something you’re already contractually obligated to do isn’t consideration. You need to provide some form of new value. Of course, this is assuming that the raise is exactly the one agreed to previously. If you increase the raise amount or offer more generous terms, that would be consideration.
Hope that helps. Glenn