Acquisition/Merger contract question

BACKGROUND:
Earlier this year my company was bought out and I now work for the new company (same office, same location, same job, just new "FROM" on the paycheck). Before we were bought out, I had a written agreement (actually two; it is on two employee reviews) with my manager that I would receive an X% raise the day after I achieved a specific certification. After we were bought, that agreement was referenced in a meeting with my manager verbally stating something along the lines of "we'll still do something when you get certified".

Now for the question:
I achieved my certification after we were bought and under the new company. I finally got something 11 weeks after achieving the certification, but it was in the form of a bonus andnot a raise. My question is does the company have to stick to the original agreement or can they change their policy? By the way, they didn't have a policy in place when I got certified... that's what took so long, they were composing policy.