Update for the benefit of the archives:
I've been unable to find anything broken from my "forced" upgrade so I've this morning carried it out on the live server with (so far!) no ill effects and only about 15 mins upgrade time (for most of which the server was online).
I'm pretty sure that the "correct" route would be to download the old release CD onto the server, loop-mount it, and upgrade from there, repeating through each iteration needed to get things up to date. However, having already done it the "wrong" way and had it work, I didn't see a benefit in re-testing everything the right way.
Hope that's of use to someone one day.
-- Mark Rogers // More Solutions Ltd (Peterborough Office) // 0844 251 1450 Registered in England (0456 0902) @ 13 Clarke Rd, Milton Keynes, MK1 1LG