I also noticed that, and take note signature campaign manager are not called bounty manager, IMO bounty manager are those who pay in altcoins or tokens during the ICO, just to make it more accurate.
I've been in the forum since 2016 and I never see this things happening before, this simply means that there's a shortage of campaign because there are a lot of accounts looking for signature campaign, these would not happen if bounty is still profitable but unfortunately it's not anymore.
So, to answer the question if it's legit or not, I guess there's no problem with that since it's not against the forum rules and the manager can make it's own rules, revise the existing one at any time he wish to do, of course as long as in conformance with the forum rules.
Now, I understand your sentiment that low rank accounts that are already in the campaign and those who are applying might loss their opportunity to be part and to upgrade in pay in case a spot opens up. IMHO, there is a disadvantage for the high rank also since they will receive a lower rate compared to the rate they are supposed to, so it's an even game.
I would to site you an example, just recently
bustadice signature campaign(FULL) had an opening of a member rank but instead of hiring a member,
yahoo62278 did hire a
Legendary for a 25$ per week which suppose he will be receiving 100$/week at the current rate, and in
his past campaign he is receiving
BTC0.015 per week which is equivalent to
$58 and that's even half of what he is receiving now.