I need to build a new rear hub so have a few questions, firstly, how easy is it, but more importantly, can i use the same spokes, the current rear hub is a shimano m454(or something, can check if needed) and im looking to buy a superstar switch rear hub and am lacing it to a mavic xm117 (same as was on shimano rear hub). Do i need to buy a new set of spokes?

Thanks a lot,
Sam

Probably yes, unless the PCD & flange-centre's are the same on both hubs.
Always advisable to have new spokes in a new build anyway.

Its pretty easy. But buy some new spokes for £15 off crc

Depends, if the length of the spokes req'd is within 1-2mm of each other then it's OK, and if you know the old spokes haven't had a battering, then I'd re-use them. If in doubt use new spokes.

they're less than a year old, how do i know which length of spokes is required?

Technical pages on Shimano website will tell you, as will the superstar website. Alternatively if you know how to take those measurements yourself, there are any number of spoke calculators on the internets

YTour existing spokes will be fine if the rim is the same and the flanges are <10 different in radius*. The PITA is that they will be "set" to the pattern they are in** and it's best (albeit not essential) to replicate that pattern in the new build.

*spoke length varies by 1mm for every 10mm change in flange diameter

**i.e. exiting flange on outside or inside.

how do i know which length of spokes is required?

You'll need some details of hub & rim such as: ERD of the rim, pcd of the hub flanges, hub flange's-centre, number of spokes & how many cross pattern.
Have a look at the wheelpro website, one of the easiest to use.

SO, the shimano hub is
Hub flange diameters – 61mm (58mm superstar)

Flange distance -35.35mm(left) 21.75(right) (superstar 36mm left 22mm right)

Will they be fine?
(remember its the same rim)

See my post – yes – I'd caveat this by stating that (i) if the spokes are not good quality they may snap after the re-build and (ii) check they ain't seized.

they were built by merlin originally so presumably are decent quality,

thanks for the help,
Sam

IMO wheel building is not something to take lightly. It took me 2 or 3 old 'practice' wheels to get it right. I would STRONGLY recommend you have a swear at an old wheel before trying it on your new bits of kit.

… also refer to Sheldon.

Where are you from Spock? Happy to give you a lesson on the basics if you're not too far away