I don't have enough time to study the solution by myself and I'm not experienced with sed
....and, by implication, you also don't have time to learn SED?
This is not a good way to get help here....
First, you don't say what the criteria is for making a change. For example, is it always the last portion that needs to be changed? Do we need to test and sure the target number is part of a MAC? ....etc.
Second, why SED? If you set the the field separator to ":", then AWK is very handy for discriminating columns.