Like Geo, I've done it and it seemed to work well enough for me. However, I had the brakes apart for other reasons later on, and discovered some black tarry goo in places that I believe was the remains of the old DOT 4 fluid. It does not mix with DOT 5 at all, and is more dense, so I believe any remaining droplets of DOT 4 sink to the bottom and lay there sucking up the dye from the DOT 5 as well as any dirt, wear particles from the seals, etc.
Another anecdote (that doesn't prove anything) - On Stag #1, both seals in the brake master cylinder failed nearly at the same time, just a few months after converting to DOT 5. When I got it apart, the cups were much softer than usual (about like al dente pasta) and had torn apart between the base & seal surface.
And yet another concern : The hydraulic brake light switch used on TR3s sometimes doesn't seem to like the DOT 5. After converting my TR3A, the apparently original switch stopped working within a year; as did the reproduction from Moss; as did the similar switch from FLAPS (originally fitted to some old Ford pickups IIRC). Other folks have reported not having such problems, so maybe it's just the luck of the draw or whatever. And I can't really prove it was the DOT 5 (I also had higher power brake lights on the TR3A), it just seems suspicious. But I finally converted to a mechanical switch. Even the first mechanical switch on my TR3 gave me headaches, which in retrospect I blame on DOT 5 leaking from the reservoir, down the pedal and wicking it's way inside the switch. Fixed the leak, replaced the switch, it's been fine for about 4 years now.