Just got their (I should say step-mom's) reply:

The reason I tried to "earmark" it is because it is easier to send a certain amount of money occasionally then more a month. Also in J's eyes it makes it more understandable to him. You have to understand that he gets a little unsettled each time a review comes up simply for the fact that he sees N only 8 weeks or so out of 52 weeks, and is sure that the 8 weeks will start to dwindle as N gets older. Their relationship is barely there. Distance makes it hard to have a relationship. I don't say that as a put down or anything defensive, just trying to get you to understand his state of thinking and state of mind. In his eyes, he feels he tries to help out when he can by using his gas to go to the airport in Chicago and calling around trying to get cheaper tickets, and such. (**note, that this has only happened one time which is this year) He panics when the word "raise" comes around. After discussing it with him, I think we have maybe come to a reasonable agreement that is easier to obtain. Would you be willing to keep things like they are at the moment, but then get the lump sum of $600 at tax time each year? That way J isn't worrying every month about additional monthly bills, and yet you are getting what you need for N growing needs. So the agreement would be in affect from the expiration date of this one until 2 years and would state everything it currently says except with the addition of $600 at tax time. That way nothing additional is being purchased and you have the money to spend as you feel fit towards his growing needs, and it's not so difficult to obtain for J. Basically a win win for both of you. Does this sound ok to you?***

I'm not sure how to reply.
I didn't want to do this now...our active agreement is good 'til January 2010, so we are talking about agreeing to something almost 2 years ahead of the renewal and with rising costs. This is advantageous for them, to "lock in" so to say the child support until our son is 17 but I don't see how it benefits me (as it may even be to a disadvantage) to agree to this future change NOW.