Do the research and run the numbers through the Idaho online CS calculator with a range of income snenarios for BM.  Significant change of circumstance usually does not include periodic promotions.  Things like winning the lottery or a massive increase in income are qualifying events.  Unless your DH got a 50%+/- raise I don't think you have much to worry about. Besides, DH can delay and postpone until the major event of the last SKid graduating from HS.  The only think that may postpone that is if the kid graduates before turning 18 in which case they will age out on their 18th B-day.

There is nothing magic about Sr. year other than CS in most states ends at the end of Sr. year upon graduation or the Skid turning 18 whichever comes first.

A good friend of one of my childhood BFF's won the lottery. He had to pay a massive lump sum in CS each year when he received his annual lottery pay out.   Sadly the winner was not a great money manager and he and his wife usually ended up on welfare for the last few months of each year because they ran out of money.  The BM never had to deal with that since she got her money up front each year.

I got this from the Idaho CS mod web site.

https://www.acf.hhs.gov/sites/default/files/programs/css/id_cs_order.pdf

  Unless major changes have occurred, CSS will not review child support orders until it has been three years since the order was signed, changed, or last reviewed.   For CSS to provide modification assistance, the review must show that the monthly child support order amount should increase or decrease by more than 15 percent and by at least $50.

Plug the old info for BM income into the calculator and see if DH's rraise impacts his CS obligation by at least 15% with a minimum hurdle amount of at least $50/mo.  Then you will have an idea of whether or not BM has a chance of gaining a CS increase. She can threaten all day but if she either does not file or the CS review office does not recgnize a significant change in circumstance (since it has been less than 3 years since the last review) she is SOL.

So, get your ducks in a row and get ready to smack the shit out of BM in court if she steps out of line.  I usually find a run for money in CS form to be rather humorous and humiliating for the person doing the filing.   Rarely does it go well unless there has been a significant change in circumstance.

In our case, when my DW filed for a CS review after 9 years CS went up for the SpermIdiot by almost 600%.  The SpermIdiot about shit a brick.  Interestingly his income had not changed much but my wife's had gone through the roof.  The CS formula does not generally care where the increase in income comes from since any increase in income increases the total resources available for support of the child.  If parenting time remains the same CS can explode for the NCP even when it is the CP's income that goes up.  SInce it has been less than 3yrs since DH's last CS review the risk is low that BM will be successful even if she tries it.

The time left before the SKid ages out from under he support CO makes this little more than a last ghasp for BM to dig into DH's pocket for a few months.  

Make it painful on BM. Don't give her any information. Keep her in the dark.  That way you can unload on her when it best suits your case.

And have fun.

All IMHO of course.

Ugh. I'm sure she can do it, and here, she would get the increase. These greedy BMs don't like giving up control over the ex, so why not drag them back for one last court visit. 
 

I doubt the loan will factor in, it wouldn't here. CS and tuition are separate.  Take heart, at least it's ending soon. 

Sounds like she has every right to request it be evaluated. Yeah it's annoying but that doesn't matter. Let the courts handle it. If you don't want to drag it out then give them the information they need and move on.

Is the step going to college of a trade school? If so, BM could be looking to have CS continue.