Replacing the collar as well as this RubberBush would make sure the results are 100% - Mixing old parts with new can cause premature wear of the new item!
Rubber items like these tend to degrade over the years, through general wear and tear and of course U.V exposure, so take the opportunity when rebuilding your forks to replace these at the same time.
At this low price why put up with the worn out old items!