The following is from the UIFSA. My comments are in bold.
SECTION 201. BASES FOR JURISDICTION OVER NONRESIDENT.
(a) In a proceeding to establish, or enforce, or modify a support order or to determine parentage, a tribunal of this State may exercise personal jurisdiction over a nonresident individual [or the individual's guardian or conservator] if:
(1) the individual is personally served with [citation, summons, notice] within this State; Since you're in SC, I'm assuming this one doesn't apply to you
(2) the individual submits to the jurisdiction of this State by consent in a record, by entering a general appearance, or by filing a responsive document having the effect of waiving any contest to personal jurisdiction; Only you know if you've done this one or not, see my last comment at the end
(3) the individual resided with the child in this State; Did you do this?
(4) the individual resided in this State and provided prenatal expenses or support for the child; Did you do this one?
(5) the child resides in this State as a result of the acts or directives of the individual; How about this one?
(6) the individual engaged in sexual intercourse in this State and the child may have been conceived by that act of intercourse; Since you say she was conceived in germany, I"m assuming this one doesn't apply. But are you SURE about the Germany part? [or]
(7) [the individual asserted parentage in the [putative father registry] maintained in this State by the [appropriate agency]; Did you do this? or
(8)] there is any other basis consistent with the constitutions of this State and the United States for the exercise of personal jurisdiction. I haven't read the Alabama laws, so you'd have tor esearch this one yourself
(b) The bases of personal jurisdiction set forth in subsection (a) or in any other law of this State may not be used to acquire personal jurisdiction for a tribunal of the State to modify a child support order of another State unless the requirements of Section 611 or 615 are met.
***Now, Section 615 wouldn't apply to your situation as that pertains to modifying orders of a foreign COUNTRY or POLITICAL SUBDIVISION (Such as Guam, etc.). Your order was issued in Illinois, which isn't a foreign country or a subdivision thereof, but a state in this country. So, Section 611 becomes the controlling factor for you.
SECTION 611. MODIFICATION OF CHILD-SUPPORT ORDER OF ANOTHER STATE.
(a) After If Section 613 does not apply, except as otherwise provided in Section 615, upon [petition] a tribunal of this State may modify a child-support order issued in another State has been which is registered in this State, the responding tribunal of this State may modify that order only if Section 613 does not apply and if, after notice and hearing it, the tribunal finds that: **Section 613 doesn't apply to your situation, as that pertains to when ALL parties reside in one state, and you don't. She's in AL, you're in SC.
(1) the following requirements are met:
(A) neither the child, nor the individual obligee who is an individual, nor the obligor do not reside in the issuing State; This burden is met. None of you live in Illinois (the issuing state) any longer
(B) a [petitioner] who is a nonresident of this State seeks modification; This one doesn't apply as SHE is the petitioner and lives in the state and
(C) the [respondent] is subject to the personal jurisdiction of the tribunal of this State; See above or The word OR might get ya here
(2) this State is the State of residence of the child, It is in your caseor a party who is an individual is subject to the personal jurisdiction of the tribunal of this State, and all of the parties who are individuals have filed a written consents in a record in the issuing tribunal for a tribunal of this State to modify the support order and assume continuing, exclusive jurisdiction.
(b) Modification of a registered child-support order is subject to the same requirements, procedures, and defenses that apply to the modification of an order issued by a tribunal of this State and the order may be enforced and satisfied in the same manner.
(c) A Except as otherwise provided in Section 615, a tribunal of this State may not modify any aspect of a child-support order that may not be modified under the law of the issuing State, including the duration of the obligation of support. If two or more tribunals have issued child-support orders for the same obligor and same child, the order that controls and must be so recognized under Section 207 establishes the aspects of the support order which are nonmodifiable.
In a proceeding to modify a child-support order, the law of the State that is determined to have issued the initial controlling order governs the duration of the obligation of support. The obligor's fulfillment of the duty of support established by that order precludes imposition of a further obligation of support by a tribunal of this State. This is very important to you... re-read what's in italics
(e) On the issuance of an order by a tribunal of this State modifying a child-support order issued in another State, a the tribunal of this State becomes the tribunal having continuing, exclusive jurisdiction.
Basically, in a nutshell.... Alabama "may" have jurisdiction to modify the current Illinois order, HOWEVER, they have to use Illinois guidelines to modify. For example... if CS goes to 18 in IL, and 21 in AL then AL has to follow IL guidelines and end support at 18... not continue it until 21 like AL law allows for. Likewise, if support goes until 21 in IL and ony 18 in AL, the AL court can't modify the support to terminate at 18 like it does under their laws. The same thing goes if the original order is modified in SC. SC would have to use IL guidelines to modify the order, they can't use SC guidelines.
My question is this though.... Did you, AT ANY TIME, answer an Alabama petition? Like, when the petition to first modify this order was served on you? If you did... did you immediately contest jurisdiction in AL? If you did not do that, and just answered the petition with the "standard" response, then you may have unwittingly submitted yourself to Albama's jurisdiction.
Honestly, since your objection raised in your first post was "Ex-wife is taking me to court to increase child support amount (subsequently this would also extend the number of years I pay - different state law)", is a moot point under UISFA, why wouldn't you want the support modified in AL? I've shown you the law which says the issuing state's laws are controlling in a modification... so you wouldn't be paying "longer" if it was modified in either AL or SC.