A long time ago a grizzled old friend of my dad taught me a trick to attach a tippet to fly line, without knots.
Trim the fly line above where the old tippet attached. Take a needle an heat it with a candle. Once red hot, Insert it into the center of the end of the fly line, this will melt the inner core, and expand the outer core slightly. It might take a couple times to open it up enough for the tippet. I like it to insert at least 3/8 to 1/2" into the tippet.
Once you can insert the butt end of the tippet far enough into the line, remove and put a drop of super glue on it, then insert into the fly line and let it dry. I've yet to ever have a line seperate after doing this. There are no knots to snag on anything! Works slick.
-DallanC