When I last resealed, I cleaned the area with Rubber roof cleaner, then scraped any loose old stuff off carefully with a plastic scraper, making sure not to gouge the rubber membrane.
It is not necessary to remove all the old Dicor, so if you can't easily remove it don't. Then fill the separation. I have overlapped over the old just to make it look nicer.
You also may want to think about
Eternabond, I plan on resealing my roof with this next time. It is expensive, but many members of this and other forums have used it with excellent results, and it lasts almost forever.
Stewart