I upgraded my focuser to a Crayford Duel Focuser last year, this is how i did it, you will need to make the hole on the side of the Optical tube assemble bigger, first I removed the rear mirror, after that I put the scope tube on it's side, leave the correction plate ( lens) on the front it will keep the tube ridged, before you start un-screwing the old focuser place a old towel or some material under the focuser and in front of the correction plate (lens) this will first stop the nuts from dropping on the inside of the tube and scratching the mat black paint, and second the nut wont bounce off the tube and scratch the front correction plate. You my need someone to help you with the nuts inside the tube from spinning, they will need to grip the nuts with a small set of long nosed grips/spanner, at this point keep the towel in front of the correction plate just in case they slip and hit the correction plate with the long nosed grips/spanner.

Here come's the nervous part, once the old focuser is off you will need to cut the metal on the hole to fit the draw tube of your new focuser (i used some metal cutters), i basically cut 5mm cuts outwards into the side of the hole at 5mm spaces all around the hole where the draw tube will go, once you've done that fold the metal that you have cut inwards (I could of used a drill with a small grinding tool on it but TBH i was scared any vibration my of cracked the correction plate or secondary mirror or if i slipped i could of damaged the main tube, so not recommended), some of the paint will drop off but the towel will catch most of it. Now you have done that you should be able to see the draw tube of your new focuser go throw the hole, you will next need to align the focuser with the secondary mirror, you can do it with line of sight, but a Cheshire Collimation will get it perfect, with a pen mark off the holes that mount the new focuser on the main tube, then drill the fixing holes to the size of the bolts you are using, then you can mount the new focuser, yet again this is where a second pair of hands will help as the nuts will spin again, that's it job done, take the towel out, you will get most of the metal shards out, but i used the hover with a brush end to remove the rest of the bits of metal/paint. You can now put the main mirror back in place, it will be useful before removing it in the first place to mark the tube and the outer black casing of the primary mirror so it goes back in the right position it went in, then all you need to do is collimate the scope. You will also need a 2 ins 45mm extension tube as these focusers are low profile ( there about £16), if you dont fit one you will not get into focus with your eyepieces or a CCD if you have one.