Hi jp,
I replaced my speedo cable bushing a while ago when I replaced my speedo cable boot. I don't remember it being and issue to replace the bushing so I think you should be able to separate the bushing from the cable by a little bit of pulling apart/wiggling the two. It is probably stuck there by grease or dried up oil.
As far as putting the new boot on, I remember I put a little silicone grease on the boot and cable, let the boot turn inside out while pushing the boot up the cable then pushed it back down once the cable and bush is reinstalled with the earth bolt, letting the boot pull out right way up. I think it is recommended to pack the end of the cable and bushing with grease before pulling the boot back down as this helps prevent the no. 1 culprit for killing gearbox bearings, water entry through the speedo cable boot. For this reason I recommend you seal the upper boot/cable junction with a silicon sealant or similar RTV once installed...
Hopefully I haven't confused the hell out of yer now...
