Here is a technique I use when climbing SRT with 16 strand 1/2" poly kernmantle, specifically NE Rope safety blue. I use a pair of LH RH toothed Petzl ascenders with the two foot loops. I practiced this 3 ft off the ground and I didn't take it high until I could do it six times without mistakes.
The changeover:
1)Boots out of the loops, unload cam for lower foot loops and remove from line.
2) Attach Fig 8 descender to line. Verify that it is oriented correctly should you need to softlock it.
3)Attach carabiner to your center ring in front of your existing clipped in hardware (closest to you) and clip in the descender.
4)Take a big pear carabiner and attach to your right (or left depending on your descender and line orientation) side D ring.
5)Form a Munter hitch in your line and clip into the carabiner on your side D.
6)Grab your line below the Munter hitch and pull upward, tilting your body to allow as much rope as possible to pass thru from above you thru the Munter hitch. If you do this properly, the Munter hitch should come back thru the biner and reverse when you let go of the line, and assume the proper orientation for descending. It's possible to get hung up if you don't do this now. Don't wait till your full descent weight is on it to get the knot to reverse.
7) Clip your lower Petzl that attaches to your boot loops back into the line. ABOVE the Fig 8 descender. This is SO important. If you mistakenly clip in below it could jam up against the Fig 8. Clip it in close to the Fig 8, just above. If you do get it wrong, you can fix it by re-attaching the top ascender, then footlocking the rope and pulling it down.
8)Put your boots back in the loops. Using the line, and not the descender strap, pull yourself up as you stand. Now all your weight is on the lower Petzl, and with one hand steadying yourself holding the line, unload the cam and remove the top ascender.
9) IMPORTANT TECHNIQUE - Before you let go of the line and let your weight settle onto the descender, grab the running end of your line below the Munter hitch and pull it upward and hold it. This keeps you from descending and ending up with your heels over your head, all crunched up in the Petzl lower half. Take your feet out of the boot straps.
10) Now, you will prbably find that between both friction rigs, the Fig 8 and the Munter, you won't go anywhere if you take your hand off your line. So at this point, I take a moment to unwind the Petzl straps out of my center ring and climbing line and let it hand off to the side where it will not entangle with anything else.
11) Hand feed your line up into the Munter, and you will descend a little bit for each length you feed in. If you have bare hands don't let your fingers get nipped by the Munter. That's it. I keep my legs straight out and my toes up and use the descent as an opportunity to stretch my calf muscles.
It is possible that if you use a rope other than what I use, it may slip thru more easily, but I find that I move hardly at all unless I feed the line into the Munter.
Get hitched!