I've done this with some old lines as well and as the others have said, it will work just fine. On intermediates, I usually use 0X fluoro for the trap nail knots rather than Gel Spun as seen in the video. This keeps the loops nice and clear like the rest of the line, assuming it is a clear intermediate.
Also, pay attention to which part of the line you are harvesting from. The belly section will be pretty fat which may make for some thicker than desired loops. The running line will be nice and thin, but may be a bit light, lacking turnover when casting heavier flies If you get a section from the back part of the rear taper transitioning to the running line, you'll get a mildly tapered sink tip that will roll out nicely.
Have fun tinkering!
Dan Davala