It depends on how you interpret the take over.
I believe that it's Marcy's body with the core as the controlling mind, which would make it technically still marcy, who uses She/Her pronouns
If you see it as the core with Marcy's body, (which is what I think is technically correct since the Core is the one in control, just using her body as a vessel) then you would be referring to the core, who is a They/Them.
Marcy and Darcy are She/Her, the core is They/They from how I see it.
(Feel free to reply but I'm going to sleep so I'll see it tomorrow)