It is so easy. My medium FBs fit my ds from 4 months until PT (3 years old), but after years of washing every few days and using bleach pretty regularly, the elastic is pretty shot. I am replacing it for my new baby (who is not a baby anymore almost!!!).
All I do is get my trusty seam ripper and rip out the elastic casing (I do this from the inside of the pocket on the microfleece side, to prevent tearing the PUL). When the casing is open and the diaper is completely inside out, I then take out the old elastic with my seam ripper (it makes the seams less bulky). Then I use 3/8 Stretchrite knit elastic and sew it down at the starting point with a zigzag stitch, then pull the elastic as much as I can and 3 step zig zag it down. I zigzag it down at the ending elastic point (both starting and ending points I marked with a sharpie BEFORE removing the old elastic) and cut the excess elastic off. Then I turn the diaper right side out and sew the casing back where it was, pulling my elastic taunt and trying to not catch it in the casing. I also use poly thread to prevent wicking.
Now they are better than new since my elastic is a bit more elasticy (not a word I know) and I get fewer leaks out of the legholes.
BTW, my PUL, microfleece, and snaps are still going strong after 3.5 years of constant use. I CAN'T kill those diapers if I want to. I have 18 mediums and they all still work and look new from the outside.