Posting in case the info is useful.
In May I reserved an appointment at the Jlm embassy for August 2nd to renew my 4-yr-old's passport. In June I reserved an appointment for December to renew my 9-yr-old's passport, when I belatedly realized hers was expiring too.
I've been checking since July to see if there was an earlier appointment to change the latter one, because it is worth good money to me to have it in hand before the end of the year. Not only were there no earlier appointments, I haven't see any at all available. (I remain mystified regarding how easy it was to book those two, because now, there seems to be nothing at all.)
Anyways, I came across an option to "place a request to combine two existing appointments." Having nothing to lose, I requested that the December one be moved to August. This was Wednesday. Today, I got an email confirming that the request was accepted!
I hope this info is useful.