When I changed my Imp to DOT 5, I pulled each cylinder and cleaned the system out with some solvent. Solvent like Xylene readily evaporates but will clean out any residue of the old fluid when blown clear with compressed air. The remnants of the old fluid in the wheel cylinders is what I was worried about. The hygroscopic nature of the traditional brake fluid is what we’re trying to avoid in classic cars which aren’t used daily.
(For the benefit of all who don’t know DOT 5 entrains air, so never use a vacuum to suck it into the system. Always push it in for best results to avoid spongy pedal.)