yeah you can swap entire forks over between 500/550s,the first 550s were only a 500 with the 550 engine and those "improved" front forks,unless someone knows for sure i cant see why the guts wont interchange?you cant swap the sliding tubes though,they go with the dampers to match!often the lower leg has a different part number as it has gaitors or the simple slider seal so the upper lip is different only,ive never felt any difference between 500 and 550 front ends,in fact the 550s ive had have felt harsher?id give it a go,the caps are different as you wouldve found out for the threaded rod,use atf a bit less than the book says for a start,then go from there,the 550 right hand lower leg has provision for a second caliper that never happened from the factory,these get used for if you do fit the second front disc with a little work to the speedo drive area.