Re: "Express Replacement" : "I recently dropped my phone in some water, and for the most part, it was
pretty deep in there. Overall, the iPhone seems to be fine; it's a 16 plus. I'm worried that in the future this may cause some damage, although nothing seems wrong now."
See if Apple Support's online article (link below) can help:
ie: Look within the article to see the table, and the illustrations
Water and other liquid damage to iPhone or iPod isn't covered by warranty - Apple Support
Eg: says
"To find out if the LCI in your device came into contact with liquid, find your product in the table below and locate the LCI or LCIs in your device. To help you see the LCI, use a lighted magnifying glass and angle the light or the device until you can see the LCI.
This table shows where the indicator (or indicators) are and how they look after being exposed to liquid:"
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Eg: article also says at the bottom:
"If you aren't sure if liquid exposure is causing an issue for your device, even if you see a red LCI,
contact Apple or an Apple Authorized Service Provider to inspect the device and provide assistance."
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Re:" If I were to do express replacement, and Apple found that the iPhone I sent in was in perfect condition,
would they charge me extra, or would it be fine?"
We are all Apple users, like yourself, in the community,
so no-one here can anticipate what Apple may say or do in any particular instance.
You can: "Learn more about what’s covered under your warranty,
costs for repair outside of warranty, and more for your specific Apple product."
Product Service and Repair Information - Official Apple Support
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Re: I have full AppleCare, and I was thinking of doing express replacement.
You can check your Apple Care Plan Terms & Conditions: Legal - AppleCare+ - Apple
If you wish: Call Apple Support at 1-800-APLCARE (or use the app)
AppleCare - Apple