I have some and i thought that you just unsnapped the booster (and just layed it in the nappy instead) - then you can shorten the rise by folding it down at the front and using those snaps to fasten it. Does that make sense?

I'll try to take a pic but i have never been good at uploading photos to BB!

Its the only way i can think of doing it and cant for the life of me find my instruction/information thingy on them atm