User blog comment:Wrath022/Chapter 517: Wendy Belserion, Review/@comment-25111584-20170115052308/@comment-7418318-20170115085849

"It's going to be so much fun to see Mashima's reason for not allowing Irene to simply enchant herself into Wendy again, especially now that the latter is unconscious. /o/ "

Sounds like a pretty compelling reason not to do it at that moment, wouldn't you say?

If being unconscious comes from the body, rather than the mind, she would turn herself unsconscious in front of at least one clearly wakeful opponent and unless her mind stays unconscious inside of Irene's body, Wendy could just as well reverse the process again.

But this time they have all the time in the world to prepare first, for example they could quickly retriefe some magic-blocking cuffs and the second that Wendy switches, Erza can clap those on Irene before she can react.

Much better for Irene to kill Erza while Wendy is out of the picture, then capture Wendy and do some preparations of her own before the next take-over attempt instead of doing it in the middle of a battle.