If you offered a reasonable payment plan ...in writing ...and she rejected same....it might not play too well for her in court ! At least as posted you have no duty to pay it all inside of 30 days . Be sure you can prove you made such an offer and she rejected your offer.
If you made an offer and she rejected that offer then that deal is off the plate , she rejected it ....makes no sense to transfer money on a rejected offer ??
As an aside , if there was a substantial change in your pay, downward , you might re evaluate if you should petition to have CS lowered consistent with guidelines at your current pay rate.
I'd be very careful in how I worded my job change ...a change required to meet your duties to get child back and forth to school does not smell bad to me ( little that I know ) but a voluntary pay cut to suit your personal needs may smell a lot different and court imputes your prior earning capacity.