@JHUstu
Simple answer: Yes, you can file an amended tax return, claiming her as a dependent and probably claim the tuition credit and maybe more EIC. That, of course, assumes she qualifies as your dependent. And, as you apparently know, she will also has to file an amended return, checking the box that she can be claimed, as a dependent. Depending on why she filed, there may be other details that need to be amended.
Why did you “let her” claim independent? With the tax law change, effective 2018, most students will get the same refund whether they claim themselves or not. The personal exemption has been eliminated and the standard deduction increased. Did she claim the tuition credit? There's a new urban myth among college students that says they can get a $1000 from the government just for filing a tax form. For most of them, they simply aren't eligible. A full time, unmarried student, under age 24, is only eligible for the refundable portion of the American Opportunity Credit if he supports himself by working. You cannot be supporting yourself on parental support, 529 plans or student loans & grants. She can’t get that $1000, even if she is not your dependent. She can get the nonrefundable education credit or deduction, as a non dependent.
There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test. Only a QC qualifies a taxpayer for the Earned Income Credit (EIC). See full rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...
Does she meet the residence test? In order to be your dependent, she must live with you and her time at college must only be considered a temporary absence. That doesn't mean she can't use another address for mail.
The income limit for a qualifying relative is $4200 (2019). So, she may still be a dependent even if she permanently moved out and can not be a qualifying child.
If she files an amended return, she doesn’t need to change her address (it’s only her mailing address). She doesn’t need to pay back the stimulus payment, unless the IRS asks for it (unlikely), if she will not be your dependent in 2020 (she turns 24 and can no longer be a QC) "In essence, the stimulus check acts as an advance of your 2020 income tax refund. This means when you prepare your 2020 income tax return, there will be a line to include the section 6428 credit. The credit on your 2020 return is subtracted by any amount received as a stimulus check in 2020. If the amount you received as a stimulus check is less than the credit you are due, the difference will be included as part of your 2020 refund. If you have been overpaid by receiving the stimulus check, however, you will not be required to return any excess amount".
“The main reason about changing to dependent is so that she can stay on her health plan.” Have you checked with your plan. It’s my understanding they can be on your plan through age 26, even if they’re not your dependent (Reference: https://www.hhs.gov/healthcare/about-the-aca/young-adult-coverage/index.html#:~:text=Under%20current...
