He sure does like trying new things
yes, but if you look at his career, he didn't changed much
during the 60's every british player used Rotosound, andit was normal to have those. McCartney was one of those players who didn't care about his instruments, and as he said, he hated the Hofner due to lack of intonation and cliffhanger's action, so since Rickenbacker gave him the 4001 (strung with Maxima flats), he stopped using the Hofner in studio (from Rubber Soul, only Rickenbacker, except some tracks on Abbey Road and Let It Be), and at a certain point he strung it with the new Black Nylon rotos, that became a trend.
after the Beatles, he left the Hofner in his case and took it back for the Wings... he says that when he opened the case, the poor bass still had the "bassman" sticker and a paper with the Rooftop Concert playlist...
he wanted to use it only because it was his "iconic" bass, the one people wanted to see on his shoulders, ad because it was light, easy for two/three hours gigs, so he gave it to his luthiers and, and then became his main bass, with low action and Pyramid flats.
for the Rickenbacker, the story is different. at a certain point, during the late 90's or the first half of the 2000, Maxima closed, and Paul was out of Maxima sets. the old Rickenbacker had many truss rod and neck issues (in fact, on the warranty it was written to use only stock strings, or it will make the warranty expire), and they needed light gauge flats. Maxima made light flats with roundcore (like Pyramid and Thomastik).
so, when Paul was out of Maxima, he asked Rickenbacker what to use, and they suggested him the Thomastik flats (they are a recent product), being the most similar to the old Maxima.
however, Maxima came back as Optima, and after years making only the gold plated roundwound (their trademark product), now the have many new sets and the Optima Flatwound, pure nickel roundcore, like the old Maxima.
