So according to Henrik from recent streams it seems like they moving over sooner than expected. Maybe even while UE5 is in EA. They apparently have folks looking into the engine now. They will only move over if its stable enough, last thing they want is a downgrade in performance and stability
Yeah, it sounds like there's a push/pull, especially because some things that are annoyances right now due to the UE4 engine (streaming hiccups as you move through the world and weather syncing with clouds for example), they could fix in their current build, but it would require a lot of work and those things may either be already improved in UE5 (which apparently applies to streaming hiccups) or they'd need to re-code it when they went over if the system has changed (which may apply to weather syncing for example).
So I predict they'll jump to UE5 once the dust settles from release as they'll get to the point where the next things they need to fix on their list would take longer to code/re-code than just spending the time to switch and coding once.
All of that is of course based on how successful the launch is and if they can expand. If they can expand they could afford potentially to have devs spending more time to focus on the conversion exclusively (which is why Ashes of Creation can just do it for fun) as opposed to having to pull main devs off core development to work on it.