I don't care to toss good line that is closest to the spool, that never gets in the water. Well, rarely. It is just there in case it's needed for a big runner that ignores the drag. Except for some possible tight memory coils it's basically "new" line that served as backing for the actively used front end of line. I union some new line to the old then spool all of it onto a spare spinning reel spool. I then spool all new line onto the empty baitcasting or spinning reel spool. That keeps the union knot buried beyond normal casting distance on the spare spinning spool, and no knot on the new spool (for a while

).
Jim