Repetitive Changing

Hey I am dealing with a CSV in which I need to replace one thing say different names, and change them to a corresponding number. So for instance I have a file that says:

Jim
Jim
Jon
Mike
Jim
Mike

and Jim corresponds to 6, Jon to 2, and Mike to 3. When I am done with the file it needs to be

6
6
2
3
6
3

Okay so doing it once isn't a big deal. Each new spreadsheet only has a pool of like 20-30 different "names" to choose from ut It's a cumulative spreadsheet that I have to get a new one with all the previous changes off of it and then it has more on it. So I was wondering if there was a way that I could just do something so that it automatically changes them all in there without me having to constantly do Find + Replace All.