I have actually had this on my to-do list for a long time as about Ginger's first birthday many nappies started to give way, but as with most tasks which I am not sure how I will go about doing, I put it off indefinitely. Anyway, a new baby is a good motivator to finally tackle those jobs.
It actually turned out to be pretty easy. However, so that you avoid an entire evening of frustration and countless wasted hours of unpicking - I give you my top tip up front. You want good strong elastic and good strong thread. I was trying to use up elastic from my stash, most of which was handed down from my Nanna. So, some elastic was obviously older than others. All the nappies done with one batch had to be unpicked and redone - consquently, I am adament that you make sure you have good elastic. I also ended up using quilting thread which is just that much stronger, normal (admittedly cheap) thread kept breaking when I pulled the elastic taut.
Anyway, I took a few pictures along the way so thought I would share my method.
First, I used a quick unpick to score a small entry and exit point at each end of the existing elastic casing. There is no need to remove the perished elastic, just insert your fresh piece. I secured mine at one end with a safety pin. I pulled taut and then sewed with a wide zig-zag stitch, back stitching at start and end for security. Repeat for the other side. I then trimmed the elastic even with the entry and exit points and covered the snipped area with a tangential small zig-zag stitch.
A whole basket of re-elasticated nappies!
Ah, that makes an expectant mother happy.