Conditional Formatting based on Adjacent Cell

I have thousands of account numbers in Column A.

Some, but not all, of the account numbers need to be changed.

I wrote an "=if" formula that looks up the value of Column A, and changes the number appropriately.


*What I would like to do is highlight ("fill") the cells where the account number has changed.*


I know I can take Column A data, enter my formula in Column B to change values where necessary, and then add a Column C and use another =if function to detemine if the value has changed, but highlighting each cell one-by-one would take entirely too long in this case.


Thanks in advance, guys!

Highlight the numbers in the new Column B, created using my =if function