Well, to "go back" you'll have to do a full "re-install" of the OS.
As far as the photos are concerned...
IF Photos (10.13.1) has "updated" the library, Photos (10.13) may not be able to open it any longer.
The "way around" this might be to EXPORT your ENTIRE photo library somewhere, and then re-import it after you do the re-install.
There is a "digital photography" forum here on macrumors, you should check there or perhaps report this question there.
Hmmmm... something else to try first:
- open home/pictures
- RIGHT CLICK on the "Photos" file icon (it's more than a file)
- From the contextual menu, choose "show package contents"
- Inside, you'll see a folder named "Masters".
- Make a copy of the "Masters" folder on another drive (USB flashdrive?)
- AFTER you are reinstalled, you -might- be able to move the Masters folder to Photos file in your "clean installation", REPLACING the existing one.
(the above will work only if you keep your pics "inside" Photos)
You may lose any edits you had, but at least the pics will still be accessible.
IMPORTANT:
BEFORE you do this, do this:
- Mount the drive icon (that has the Masters file) on the desktop
- Click on it ONE TIME to select it
- Type "command-i" (eye) to bring up the get info box
- At the bottom, click the lock icon and enter your administrative password
- Now, on the left bottom in "sharing and permissions", put a check into the box "ignore ownership on this volume"
- Close get info
This works around permissions problems.
A suggestion that won't help this time, but could help NEXT time:
Next time before you upgrade, use either CarbonCopyCloner or SuperDuper to create a bootable cloned backup of your internal drive.
Then... if you DON'T like the results of the upgrade, it's trivially easy to "get back to where you once belonged" by doing this:
1. Boot from the cloned backup
2. Re-initialize the internal drive
3. RE-clone the backup BACK TO the internal drive
.... and it will be "like you never left"...
