kuuderes_shadow said:
With regards the former, imagine if, say, pool #8415 had an image with Samidare and Yuubari doing an age_switch. In that instance, both characters would still be older than their usual state, and noone younger. But this implication would tag the image with younger as well as older.
In the context of the doujin, though, Yuubari would still be "younger" so it's not that the tag would be invalid IMO. The "older" tag is there regardless so someone looking for a 20-something Yuubari would still find it in a tag search. There's some added noise for someone doing a yuubari_(kantai_collection) younger search and looking for a loli Yuubari, but that already comes with the territory with there being multiple characters or differing levels of apparent age reduction (the latter can sometimes be helped by extra tags like teenage, child, toddler, and baby). I just don't see this kind of scenario happening at all anyway, though.
With regards the latter, the wiki for alternate_breast_size states "If a character's canonical breast size is ambiguous... do not use this tag." However, a doujin could give the characters set (and different) breast sizes, which could then be switched. This implication would then tag the characters with alternate_breast_size.
That's to prevent the tag from being overused when there's no context. I do agree with that. But when it is clear that it's an alternate breast size from the context (which it necessarily has to be for a breast size switch), there should be no barrier to applying the tag.