One might think of Recovery containing a “map” for the ROM to follow.
The hypothetical of the dirty upgrade to T was to follow the Install instructions (e-Recovery + ROM) without the format data as long as Vendor parts are included in the ROM.
- Edit. (On some devices the existing e-Recovery is competent to do an upgrade. In this case one should ensure that the next update will “Update recovery”. On other devices the new ROM is slightly changed such that only an upgraded, matching e-Recovery will work.)
For some phones it may be significant “is the phone still under manufacturer development”. If going from T to a15 one might research whether the Samsung firmware still receives revisions by check the Vendor SPL shown currently on the phone with T with the dates of firmware revisions available in Stock.
I made a quick search but don’t find a reference. My memory was that there were reports that beyond* users did find upgrade Recovery necessary even with one Android version upgrade.
One might keep a screenshot of current Recovery (but it may give little clue whether you are still running e-Recovery from your e-3.0.4-t.) … edit but it may help inform whether you have A/B slots and if / when a change occurs should you run into any other difficulties.