Well, if it doesn't run unmodified, then you obviously will need to change it to get it to work!

You will need to look at the two datasheets very carefully side-by-side to identify the differences.

Then go through your code very carefully looking for anything that uses or is affected by any of these differences.

In particular, pay attention to SFR bits that may be unused on one but not the other...

Also check for any timing differences.

It might be easiest to start with the smallest possible subset of your application, then add stuff until it stops working - that should help you to narrow down where the problem lies...