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Before anyone jumps the gun; Yes, you could replace members generally by selecting the specified dungeon, and hitting "Join Party in Progress".
The question is why this doesn't work sometimes; I had a healer leave in my levelling roulette, and we waited for a good while trying to find a new one before disbanding it. I had requested in my FC (Others, Likewise) for someone to join it, but they couldn't find my party(And kept hitting a new one). Why is that? :/
Sometimes while in the duty, the person that is auto selected as leader will forget to hit the "Look for new member" option once someone leaves. Believe it or not, it's very easy to click that window away and never notice.
If that isn't done, then it doesn't put your party up for selection in the Duty Finder and you'll pretty much be sitting there forever. So long as it says "Looking for members" under the dungeon name in the Duty list, then it's got your dungeon up in the queue looking for someone to join it.
Often times however, friends or FC mates will end up being thrown into another group that may just happen to have the same role missing as your group (hate when this happens as tank) so there isn't much you can do about the situation. xD
probably there are waiting dozens of people for a good while to start the dungeon who are more likely to get a healer instead of the group wich is already in progress but didn't waited thaat long.
plus if you activate the join in progress button it is more likely to get a fail group where someone left and you will probably missing tomestones from first or second boss. so it is not that attractive to activate the join in progress button.