You can amend the form all the way until the case is adjudicated.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter,not should it be viewed as establishing an attorney client relationship of any kind.
The existing Affidavit of Support from your father in law could be withdrawn at the time of your I-485 interview, and a new affidavit of support from your aunt (with all supporting documents) could be presented at the time of the interview. You would not be "breaking" any rules! Make sure to update your address within 10 days of moving by electronically filing a Form AR-11 Address Change notification at www.uscis.gov and keep a copy for your records.
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You should update your address with USCIS using form AR-11. As far the interview goes, the interviewing officer will review your application with you, including current address and that is your opportunity to inform him or her that you have moved. You are not breaking any rules so long as you take the appropriate steps to notify USCIS.
Yes. You can file an AR-11 form electronically. Alternatively, you can call the uscis customer service hotline listed at uscis.gov. A caution,however:uscis is a giant government agency that generally is slow to change. It is better to maintain a constant address until your matter has concluded to avoid delivery problems. I say this in spite of the new tracking policy implemented by uscis. You should also know that if you hire counsel, and counsel has submitted a properly completed g-28 form, your hired counsel will receive a copy of all uscis communication. Counsel will not, however, receive an actual card on your behalf.