to put it in the most simplist of terms, morphing is the process of changeing from one form, useally larvae, to another, adult. Losing gills, the top "fin", are some of the more note worthly changes. i also have a developing theory that there are behavioral changes that happen as well, distinct changes in fight or flight response.
There is no set time frame, sometimes it can start and go slow, taking months, and sometimes days.