You do not "HAVE" to change anything besides your ebrake cables.

Just install the rear hub/trailing arm systems & run integra ebrake lines to your ebrake....

Bleed your brakes the proper way & you should be ok

The best prop valve for a rear disc setup is a 40/40 prop valve...

I say you try leaving the systems you have in place alone & run the disc setup with the proper ebrake lines. If you dont like it, just change out the prop valve later.