<p>So just to clarify, this is a complicated issue because there are at least three concepts in play:</p>
<ol>
<li> Personal Jurisdiction</li>
<li> Modification of an order of support</li>
<li> Duration of an order of support</li>
</ol>
<p>As noted by several other posters, since none of the parties reside in the original forum state, Illinois (as the residence of the father/respondent) is the appropriate forum state under UIFSA for a modification of a support order.</p>
<p>As the appropriate new forum state, Illinois is authorized under UIFSA to modify an out of state support order in accordance with its state law. So, if OP’s mom wants to increase her monthly support payments, an Illinois court is the appropriate forum and it would apply Illinois law in deciding the amount of support.</p>
<p>HOWEVER, there is a big difference between MODIFYING the amount of support and EXTENDING the duration of a support order. </p>
<p>So, if the Florida order says OP’s dad pays $X a month until OPs 18th birthday or graduation from high school - that’s it. (If the OP is only 17 and going to college, an Illinois court would likely be able to give you a semester of costs until you turn 18.) However, under UIFSA as noted above, the new forum state (Illinois) is prohibited from EXTENDING THE DURATION of support. In addition to Marshak, there are cases in other states with similar results. </p>
<p>It goes both ways - there is also a case from New Hampshire, where an order to pay college costs to age 21 from Mass. was enforced even though such payments are specifically prohibited under NH law. (New forum state can’t change the duration of the original order.)</p>