i'm going to assume that by this you mean,
what if he gave up being a rockstar to do just classical...as opposed to some wierd, alternate reality wherein he never made any rock music and had only done classical to begin with...
in which case...i would probably youtube (or find some way to listen to) each piece before i bought it...and, epending on how i felt about the individual piece: that would decide whether i would get it
i love classical music, especially music of the baroque era...on up through beethoven...on into mahler, with a smattering here and there after (and, for that matter, before: i do like a bit of earlier styles, as well) that...
some of pauls stuff i like quite a bit...others, meh
all of it is worthwhile, but, if my scarce entertainment dollars would pit a mccartney piece against, say, a beethoven that i do not have (i still have yet to get any of the piano sonatas or concertos

) it'd be tough, and ludwig would probably win...
okay - would
but, i do find his attempts, so far, to be of value and interest: enough so that i would not dismiss a career change...though, realistically, i would both not think that possible (look at billy joels attempt to tery that: i don't think maccas ego would allow that) and more likely that he would just alter his style as it is into something that he would find more artistically appealling
you wake up...sip hot tea...mindless music, radio free. see blue skies and think of sea...how are you doin'