Originally Posted by
Porcell
No. Once you kill a boss on heroic mode, those players are locked to that specific raid and difficulty. So a few examples: If you have a 10 man and kill the first boss of heroic 10 man ToES, and a second 10 man who kills the fist boss of heroic 10 man ToES, the next day those two groups can NOT intermingle and raid together. They can NOT change to 25 man difficulty (normal or heroic). They can, however, get a new raid member who is not killed anything in ToES. They can also get a new member who has killed the first boss on 10 (or 25) man normal. They can NOT accept a new memebr who has killed the first and second boss on 10 (or 25) normal, but that person would be able to join the raid after another boss had been downed.
Once a person has killed a boss on heroic, it assigns a Raid ID and that person can ONLY enter that raid instance.
If a person kill a boss on normal mode (10 or 25 man) then they are only locked out of those bosses they have already killed. They are free to join any raid (10 or 25, normal or heroic) as long as that raid has also killed those bosses, or more.