Because Urim and Thummim says that you deactivate and then activate a new one, you complete the action as much as you can.
Agree, but this is also a case of 'transitive may'. When you have an ability with "may" followed by multiple abilities, each ability has that "may".
So the ability on U+T is the same as "On activation, if you have a High Priest in play or set aside area, look at one opponent’s hand. You may then deactivate this Artifact and
then you may activate a different Artifact.