So Im getting a Epi off my mate and he has a seymour Duncan Alnico Pro and Jb. I was wondering are they a good pair or should I change the alnico to a 59.
It depends on what kind of music you play. I have a pair of Seymour Duncan Invaders installed on my guitar and they sound great for a hard rock metalish sound.

Although those pickups are standard on the Gothic SG, and it had a pretty badass sound t me.

It really depends on your personal tastes. If you find it doesnt quite sound right for your style of playing, then by all means change them.

good luck