If custom inline tracking tags are in use for onclick events or video, or there are multiple tags to migrate, or it is a publishing website which needs to maintain measurement certification standards. Then… a tag mapping process is needed, to assess what (and when) tags should be migrated to the TMS. This process assists in planning the firing rules and pages these rules need to trigger on.
The tag scan should be done both before and after deployment to verify everything has migrated correctly, and there are no instances of double firing code or missing pages. It is also helpful to scan the first 1,000 pages on the QA environment, for a sample test.
Tip1: A blog post explaining the tag mapping process by taginspector.com is here.
Tip2: Cookie crawler database can be used for quick mapping checks:
Tip3: Running a homepage speed test before and after an installation to confirm tags are loading at same (or faster) by recording the pixel initialisation time and initialisation step number and is also helpful. Here is an example: