Lookup Column & Automatically change Value

Hello, I am using Excel 2003 on Windows XP (please do not laugh) :)
Could someone please help me with the following:
I need Excel to automatically lookup column AM and change all instances whereby it finds text and changes it as follows:

Find: Change to:
1 J11
2 J21
201 J21
202 J22
205 J23
301 J31
302 J32
303 J33
304 J34
402 J41
403 J44
401 J51

For example: if cell AM3 currently contained the value: 202, I need the value in that cell to automatically change to: J22.

Any help would be greatly appreciated.

Kind regards,

Chris :)