The rebranding has been in the works for at least 3 years (staff talking about it), and I think everyone realised this property was misbranded from the very beginning.
It really can't be a Regency since it lacks a lot of the brand standards such as meeting space and banequeting/F&B - (while far exceeding the standard in other areas), the lack of such facilities is ironic since its a convention center hotel, but it is that way because the convention facilities aren't hotel exclusive, rather the hotel is part of a larger complex (that also includes the Aloft). At 189 rooms, its also a bit too small for a convention center hotel...
This hotel fit the profile of a lifestyle brand from the very beginning (2011) but at the time Hyatt had no suitable brands (Centric and Unbound came later, Andaz was still too new). Fees for brands such as Andaz are much higher than Hyatt whereas Andaz carries none of the advantages of the Hyatt name. For 90% of booking.com guests, Hyatt means a lot more than Andaz and they would pick a Hyatt over an Andaz if paying with own money.
A lot of the conversion properties (acquired) and misbranded properties tend to be given the Hyatt brand until they are renovated or transitioned into something else (this is true of their recent acquisitions in France and India for instance).
Since Hyatt is absolutely struggling (vs Marriott/Starwood and Accor) in UAE and India (both under SWA and India too recently had a bunch of rebrandings), I'm willing to bet concessions were made in this deal for the owner.
With 3 hotels, Abu Dhabi is now probably their strongest market in all of Middle East and India (relative to competition, they obviously have more than double the hotels in Dubai but are weaker relative to competition there).
There is an Andaz in the initial planning stages for La Mer Dubai as well.