change jobs mid dungeon/raid/etc

[Niwashi]

chances are it would mess with the DF algorithm to replace the missing members.

It would just reset that algorithm. E.g. if the tank leaves and the party leader sets DF to search for a new tank, but then their healer decides to switch to tank instead, that would end the search for a new tank and begin another search, this time for a new healer.

It would be abused and Duty finder would be terrible. I dont think how it cant be abused.

What's there to abuse? If your party is missing role X, then switching to role X just changes which role is missing. You're missing one or the other in either case, the only differences are how hard that role is to refill, or how hard it is to do without if you go shorthanded.

Well, I guess there might also have to also be some rules regarding whether you can roll Need on loot drops. I'd guess that it's the job you played during a fight that determines whether you can Need on loot from that fight. So no waiting until you see what's in the chest, then switch to the job that can Need on it (like from one DPS job to another while running shorthanded with an empty DPS slot). But so long as that rule is in place, I don't see anything abusable from a system that lets you (while out-of-combat) switch to a job whose role is currently missing from your party.

[Cnidarian]

It would just reset that algorithm. E.g. if the tank leaves and the party leader sets DF to search for a new tank, but then their healer decides to switch to tank instead, that would end the search for a new tank and begin another search, this time for a new healer..

Exactly, that resetting the algorithm is messing with it. In essence, it would probably pull you out of the queue if someone changes and then place you back at the end of the queue when trying to find a new role replacement. Give some latency delays between someone changing roles and it selecting a member (i.e. it found a member essentially at the same time someone changed roles) and you start having issues popping up like 2 tanks, 1 dps, 1 healer (one dps changed to tank at the same time a new tank joins). As I stated in a previous post, the only clean way I can see to facilitate changing roles during a duty would be under the following guidelines:

1. Can only change roles to one of the missing roles (even then, 2 players changing at the same time could happen)
2. Can only change roles if the party is not currently "looking for members"

Those 2 guidelines would make it so that you won't have a new player joining the instance during a role transition. The only issue that remains is stated in guideline 1, where you have 2 players in the current party attempting to change to the missing role at the same moment.

[Niwashi]

Well, if DF was searching for a replacement member and found one, the ability to switch to that role would have to be blocked immediately, including during those few seconds when the prospective new member gets the queue pop-up and needs to select "Accept". To the people in the dungeon, it wouldn't look as though the missing spot had been filled yet (and indeed it might not be if the new person doesn't accept). So there might be times when a spot appears available and someone in the dungeon party thinks they could change jobs, but would get an error when trying to do so.

So long as the error message that comes up in that case is clear as to why the attempt to change jobs failed, it doesn't seem like that would be a deal-breaker.

where you have 2 players in the current party attempting to change to the missing role at the same moment.

There's really no such thing as "at the same moment". Two people might both go into their character window to select another job, but eventually those requests are both going to hit the server. The one the server processes first happens, the one it gets second fails. So only the one player actually changes jobs. The other would get the same error as any other case where you're trying to switch to a job whose role is already filled.