Very few will and those that do switch will almost certainly switch back. In most cases it would require server xfers on top of the faction change. Recruiting is a pain in the ass so you would have to switch with a full group and be very proactive on recruiting. Even with all of that it wont last for many until or unless many others do it too. So in most cases mythic raiding will remain locked to cross realm while in the current tier.
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They are still trying to keep mythic raiding a guild event. Cross realm right away would chip away at that and to some degree the foundation of raiding over all.