You know, it is precisely those areas of law which are simpler and more general in their elucidation that ironically require more advice from an attorney.  You said you were having trouble "getting crisp on what a substantial change in circumstances can be interpreted as".  Well, even if you had read some of the case law you would discover that it usually only decides what FACTORS might be permissible for the court to consider, or what would be an ABUSE OF DISCRETION in the application of a particular rule.  over and over the family law caselaw reiterates the point that substantial changes of circumstances are in the discretion of the trial judge or family law commissioner.

i would reccomend looking at the docket on the 6th floor of the courthouse for support modifications and go in and try to sit in on some trials. or go to the family law courtroom at 1:30 and try to watch some support modification threshold hearings ("threshhold" is the hump you have to get over to meet the substantial change of circumstances) and listen carefully to all of the evidence and circumstances the lawyers are presenting and then listen to the commissioner's decision as to how he or she viewed the evidence.  or pay me to go over all the details of your case.

you can not voluntarily choose to be underemployed. they will most likely impute your income, again though, i need to see the orders.  a 15 % drop in your income (whatever the cause of it) will not provide, for example, economic hardship to you so that won't do it.

was your order an agreed order or litigated?