In particular, are any special tools needed? I have two old wheels, one with a split rim, one with a worn freehub so need to swap it over.

Depends on the hubs really – Hope are press fit, for shimano you need to take the axle out so you;l ned cone spanners, then a 10mm hex key. PLanet-X dog / DMR revolver hub has a press fit cover then you need a 12mm hex key. It's not a hard job to do as long as you have the right tools, access to something to tell you how to do it, and a bit of nouse.

they're both old (1993 and before) shimano hubs. are there any instructions online you know of? had a quick scout on bikeradar but nothing came up.

Can't help with specific instructions, but shimano hubs all tend to be the smae: remove axle, 10mm allen jey to remove freehub, refitting is the reverse. Remember which side you take the bolts off the axle and refit in the same way

cheers, very useful (I hope when I get round to it….)

The Park Tools website is a good resource for stuff like this…with piccies and everything!

My 1989 model Shimano freehub was just a 10mm Allen bolt as mentioned above. Lock ring and dust seal arrangement changed though between Uniglide and Hyperglide (but that's only an issue if you're upgrading, rather than straight swap).

Simples. Did exactly the same thing this morning on two Circa 2005 shimano hubs. took less than an hour, while watching TV. off out on a ride now.
Take all cones off drive side axle and remove axle the 10mm allan key and remove free hub. As mentioned park and You Tube probably have more detailed instructions.