It's actually kinda funny that you say only increasing the AR would keep its difficulty.
If you could do that, ranked, most people would increase the AR of all AR8 maps to 9 and it would make them 20 times easier to play, meanwhile the option to decrease AR to 7 would've potentially made those same maps way harder.
If you just added this with a score-reducing factor with no limitation on AR range it would be muuuuch better imo. That way you don't hurt current rankings (nobody would complain that suddenly people get rank for free on AR8 maps because they could increase it, or anything of that sort, because your score would be penalized) and at the same time people who want to change AR (which is usually people who only play for fun and don't care for rank anyway) would be able to do it freely with no limitations without throwing off the balance of osu!'s competitive aspect. I'm sure players like Millhiore would enjoy being able to set AR to lower amounts and it would open a ton of fun play possibilities (that are /harder/ because of the lower AR), without hurting anybody else who's not interested in playing like that.
(either way, it would be great if you finally allowed players to pick ARs higher than 10 without DT. Preferably with 1/3 increments, since 1AR = 150ms difference, 1/3 would be 50ms. In anything post-AR10 that 50ms makes a large difference)