If you would like to reroll your main, you should have a talk with the officers.
We do however need to pay attention to our raid composition - we do need to have a balance with both melee / range dps as well as healers and tanks.
But within reason, we will try to let everyone get to play their preferred role and spec.
We always urge people to reroll mains in off periods, where we're currently not progressing anymore.
Unless you're asked to change main by the officers, the usualy requirement to change mains will always be the minimum Heroic Raids requirements, for the current raid tier.
When you reroll, you will have our EP reduced with 80% - this is done to prevent people from rerolling back and forth between mains.