kars, as I mentioned above, loop-to-loop is the only intelligent way to swap leaders.
Of course, many fly lines don't come with welded loops, so it's good to have options. I know you hang on FFR - you will find people there who intentionally cut their welded loops from new fly lines in preference to their "stealthier" nail knots - I'm not even one of those.
I tie perfection loops on almost everything, except where I want a longer loop with the highest possible strength.
Surgeon's loop is a little bit bigger knot, easier for people to learn to tie reliably, and reported to be stronger.
Perfection loops, you can adjust the size during the knot to make them as tiny as you want, and they always lay perfectly straight in the line.
There's an extended moment of truth tying perfection loops where it may get away from you before you get it tight.
While perfection loops finish perfectly straight, the surgeon's loop finishes bent at 90-degrees until the mono takes a working set.
I just happened to be tying one last night, putting a small shock tippet on a new braided line on a spinning reel (was taking a macro of my improved Allbright, but had the camera there) -
- here's a surgeon's loop in 10-lb copolymer, intentionally made long enough to drop through a cigar cork on my salt UL - about an inch overall length.
while I had this set up, here's the smaller perfection loop in 15-lb fluoro shock tippet on a different line and reel,
still tied intentionally long enough (1/2") to loop on a bite-trace or lure paper-clip
already had this photo of a perfection loop in 12-lb fluoro next to a surgeon's loop in 15-lb braid
Surgeon's loop is also handy, e.g., if you have a complex dropper rig with split shot and needle-threaded plastic egg, and you want to save it to fish again next time - cut it off long enough to add a surgeon's loop above the split shot and wrap it in a leader wallet - matching loop in the new tag end of your leader, and loop-to-loop when you take it back out.
Again here, I'm talking about tying an intentionally long loop in 4x leader, so you can spread it with your fingers and drop the long rig through to complete your loop-to-loop
The little trout is on the loose egg hook that was threaded next to the Otter's milking egg, and the threaded plastic egg has slid up the leader to the split shot.
Bottom center of the photo is the size 22 midge dropper, which earlier in the day landed a 27" 8-lb rainbow.
Here's the Allbright knot, 6-lb braid to 10-lb copolymer (and set with a drop of CA)
(it's always amazing how messy your clean knots look at this level)