The metal company has trim/flashing for the angle transition. Should not be a problem. It will go over the top of the lower metal, and under the upper sheets.

I highly recommend metal roofs, especially on a barn.

I redid a barn roof all by myself a few years back that had old rotten shakes on it. Spent several days ripping off the old shakes, laid down new felt and screwed the metal down. Went from a barn that was about to be ruined by a leaking roof, to extending its life by several decades.

I used 24 gauge.