The answer I found I think is scattered across a lot of replies but I thought it would be best to post in its entirety. If it makes a difference, I've got an 8 (iOS 11.2) with iTunes 12.7.1.14 running on Mac OS 10.11.6 (yeah, slow to adopt...I'll do that someday). Windows users, it shouldn't be hard to use the same procedure as long as you know where your iTunes folder is located and if you don't have any odd preferences with regards to displaying file types in Windows Explorer.
1) In the Finder, go to your home folder->Music->iTunes->iTunes Music->Tones. This folder contains any .m4r files you had previously. Any .mp3 files you were formerly using for text tones, ring tones, or the like will probably need to be converted to AAC files in iTunes, then manually converted from .m4a to .m4r files in the Finder. I didn't test copying .mp3 files to the Tones folder on my phone, figuring it would be safer to stick to one file type and not add more complications.
I also moved this entire folder out of iTunes Music because it really doesn't need to be there anymore - as I understand iTunes 12.7, it simply doesn't recognize the folder anymore because Apple expects you to use the Tones Store on your phone rather than iTunes to manage ringtones. I left it the in the main iTunes folder for ease of access - this is important later.
2) To convert files, go to the iTunes menu and Preferences. In the General tab, click on Import Settings and switch the Convert To file type to AAC, if you haven't already. This has always been the secret way of changing the file type that iTunes will convert to even if you're not importing a new CD, though they keep changing the location of this setting.
3) Select any .mp3 (or .wav/.aiff/etc.) files you wish to still use as ringtones, go to the File menu, Convert, and Convert to AAC. I found it easy to move all my files into an empty playlist and then convert them as a batch, but it's not necessary.
4) Drag any newly converted files from iTunes into the Tones folder in the Finder. Change the file type suffix from ".m4a" to ".m4r" and when it asks if you're really sure, click on the "Use .m4r" button.
5) With the phone connected to your computer, select the phone itself next to the Music/Movies/etc. drop-down box and scroll down in the media tab to Tones. To be sure you aren't losing anything that did somehow transfer over, select everything here and drag it to the Tones folder in the Finder. If it say's you're overwriting things, click Skip or Stop...no need to have duplicates. Delete anything that remains in this folder (which really should be nothing).
6) Select everything in the Tones folder in the Finder and drag it to the Tones folder on your phone in iTunes. It is important to move every ringtone you want to use at once, because I did it in two batches and the phone somehow unloaded the first batch even though they still showed up in the Tones folder in iTunes. If you want to add more later, delete everything in the Tones folder in iTunes and re-import everything in order to avoid this odd forgetfulness.
7) I synced my phone at this point. It may not be necessary but I did it anyway to be sure. YMMV.
8) This is the odd part - any .m4r files you had on your old phone should show up in their original places. Despite all of the trouble caused by Apple's decision to change the way we manage ringtones, your contacts *should* be re-populated with the original ringtones and text tones as should the default tones in General->Sounds & Haptics.
The exception to this will be any non-.m4r files you were using previously. Since those files have been converted, they will need to be restored manually. Such was the case with the custom tone I was using for Reminders and a couple of individual text tones.
9) In addition to default sounds in Settings and individual tones used for contacts, don't forget about any custom alarm clock noises you were using. Even though Clock will still allow you to pull things from Music, I discovered that my alarm clocks were set to No Sound and Default Vibrate rather than the songs I was previously using. The time to discover this is not when your alarm is supposed to be going off.
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It's a long process but not egregiously so and should not result in rolling back your iTunes version, restoring libraries, or other more complicated processes. Apple kind of boned us on this but not as badly as, say, the time they changed the way iOS applies custom wallpaper images to fit to the screen vertically instead of horizontally...forever ruining the ability to use horizontal images without having to first take a screenshot.
Good luck!
