They wanted to do a complete overhaul like Mummy, but cannot afford to lose the capacity at this time, so it's like a complete refurb in slow motion, happening overnight.
MiB has kind of been doing the same thing, even going so far as upgrading the ride system/RVs piece by piece nightly.
ET's ride vehicles still squeak on the track, but I imagine that'll be the last thing they work on, and may actually require the ride to close for at least a few days. Everything else they'll keep upgrading as they can.