Yep, you're right - it's easy enough to manually overwrite existing replaygains. I just wondered if there was any mileage in being able to automate the overwriting instead, since we can already automate the writing to empty replaygain tags?
A back-story to illustrate my point (and how stupid I am sometimes) - over the last two years I actually thought I was automatically overwriting replaygains every time I added tracks to the library. I recently realised that only empty tags were being written to. So my collection now lacks consistency because other people tagged most of those tracks with different software and replaygain settings. As I understand it, for the whole replaygain concept to work effectively the analysis and tagging needs to be consistent, so now I feel as though I should manually apply replaygains with the same settings across my entire collection(?)