I don't see a pressing reason to switch. I'd consider the FT4(ed) more accurate in cases of hypOthyroidism, esp secondary to thyroid AA (problem in some dogs, particularly some breeds) until proven otherwise. I am not impressed with their numbers, and their studies were relatively small (<60 animals in each category).
Diagnosing hypothyroidism in dogs can be frustrating, but as it is not life-threatening, it's not like this new test which might be more accurate in some cases of hypOthryoidism ((except T4AA cases, and euthyroid sick) is a lifeline we just have to grab.
BTW, Idexx is pushing the test hard so if anyone's vet uses Idexx, you will be assigned this new test unless your clinic makes other arrangements.