my plan is to run a dedicated 50a line which would accommodate the
47a
(6-50 plug) piece of equipment. i then want to change the 28a piece
of
equipment's plug from 6-30 to 6-50 so that it will also fit into the
new 6-50 outlet.

In reality you will be running a dedicated 60A line, since you have to
run # 6 wire, which is rated for 60 amps. You can't run #8 wire because
that is rated for only 40 amps.

Keep in mind the 28A equipment might need a different breaker for
protection. Consult with the manufacturer. What kind of equipment are
we talking about?