I accidentally imported and edited dozens of photos in a previous version of Lightroom

After exporting the collection that you spent time editing to save that work you may want to change a few things so this doesn't happen again.

Where are you opening Lr from? Try opening from the Creative Cloud Manager at the top of the screen. Do you have more than 1 Lr icon in the dock? If yes right-click on the icon that has a black dot under it and in Options select Keep in Dock. Remove the other icon from the Dock.

Another option is to navigate to your Pictures folder > open the Lightroom folder > how many .lrcat files do you have here? Do you have a -2.lrcat file? If yes open that file. Delete the old - out of date .lr cat files once you are sure you are able to open the -2.lrcat file and everything is in order as you expect it to be.

I'd recommend that you change your catalog preferences: Go to your Catalog Preferences and change the first setting to Prompt Me When Opening Lightroom. Quit and then reopen Lr. This time you will be prompted to select a catalog.

Besides the method advised by WobertC using Export as Catalog (which is the most powerful if slightly laborious solution) there may be a quicker and easier way - NOTE apart from virtual copies / proofing copies. Transferring these does require the Catalog export / import method.

(That's still worth trying once, to demystify it, but it may often be overkill. It's very useful for the planned merging of (say) the new contents of a "travel" Catalog on a laptop, into a "main" Catalog on a desktop, when you return from a trip.)

This simpler option would be to write your edits and keywords out to the image files, which you have applied using the older LR.

NOTE Collection membership cannot be written out to file, so one workaround is to first apply a special keyword to those images, collection by collection, denoting this membership in a way that can be written to file.

Then simply highlight the images concerned and press Ctrl+S (Mac: Cmd+S). LR copies Catalog metadata to the file system.

• When you now import (Add in the standard manner) all these images into your new Catalog, all of that previously written metadata will get included automatically. You can now filter and highlight all images possessing that previously applied special keyword, then recreate such a Collection within your current Catalog (checking the option to add currently selected images).
• Or if you have already imported those images, you can use "Read Metadata from File" to bring that keywording etc into your current Catalog replacing all the currently seen image attributes (Library and Develop). The same comments about Collection membership and virtual copies still apply here.

I suggest you-

First make backups of your current catalog.

1) Open Lightroom CC2015 and the catalog where you did the editing,

2) Make a selection of the edited photos,

3) Go Menu> File > Export as Catalog (call it "Temporary")

Export a catalog |

4) Now close CC2015 ,

5) Open LR Classic and your "Classic" catalog and go Menu > File > Import from another Catalog, (This is a Catalog "Merge")

Merging Individual Lightroom Catalogs into a “Master” Catalog - YouTube

6) Choose the "Temporary" catalog.

I have not use this procedure so there may be some changes needed-

* In Step-2, you can limit the photos in the "temporary" catalog by selecting photos in the Library grid. For a Folder of photos, or a Collection of photos selected, with a Right-click, you can choose "Export this Folder as a Catalog" or "Export this Collection as a Catalog"

* In Step-5 If Classic says the "Temporary" catalog cannot be imported, then you may have to open it first with Classic and let Classic upgrade its format from CC2015 to Classic. Then go back to your Classic working catalog and re-try the "Import from another Catalog"

It might all sound complicated, but research the links and give it a go.