I HAVE GOT THE SOLUTION!

I experienced the same issue and had 1000s of Photos to merge into an existing Year/Date structure (Format: /YYYY/YYYYMMDD). All folders are already in Lightroom - The source (new imports from my Mobile, = "New") and the destination (existing Tree, = "Fotos")
I derived a procedure containing many elements from previous answers that

• works fast & reliable for many different folders to merge into the existing tree
• containing duplicate date folders and new date folders to the same time
• without re-importing metadate (which happens when syncing folders!)
• contains steps within OS (Win7/Win10 File Explorer) AND within Lightroom
• I add an explanation why I do as I do ... Here is the Procedure:

1. I Process Year by Year Note: In principle all Merge can be done at once as well, but repeating it for every year is not to much of extra work
2. I call the Source Tree Source/YYYY/YYYYMMDD (my "New" Folder) the Destination Tree Destination/YYYY/YYYYMMDD (my "Fotos" folder)
3. File Explorer: Move & Merge trees
   • Move Folder YYYY from Source to Destination (e.g. 2020 from New to Fotos)
   • Explorer Asks (Pop-Up): Confirm Folder Replace: This Destination already contains a folder named 2020. Do you want to merge the folder ... with this one? -> YES
   • Exporer shows the Progess window "Merging ..."
4. Lightroom: Update Catalog WITHOUT losing / re-writing Metadata, changes, edits
   • in Navigator Window select Source/YYYY (e.g. New/2020)
   • since it has been moved away outside of LR, Folder and all Subfolders show "?"
   • Right-Click Source/YYYY (e.g. New/2020) and select "Find Missing Folders ..."
   • In the Pop-Up navigate to the merged folder Destination/YYYY (e.g. Fotos/2020)
   • Click Folder and confirm Button "Select Folder"
   • LR shows a confirmation Pop-Up "The selected folder or one of its Subfolders is already in Lightroom. Do you want to combine these Folders?
   • Confirm Button "Merge" and the magic starts to happen! The Source folder disappears from the catalogue The Merged folders and files appear in the Destination folder in the catalogue All catalogue data (metadata, edits, keywords, ...) are preserved Thousands of images merged in a single short procedure

I hope you find this summary helpful! Cheers Wolfgang (aka Spaceman71)

I HAVE GOT THE SOLUTION!

I experienced the same issue and had 1000s of Photos to merge into an existing Year/Date structure (Format: /YYYY/YYYYMMDD). All folders are already in Lightroom - The source (new imports from my Mobile, = "New") and the destination (existing Tree, = "Fotos")
I derived a procedure containing many elements from previous answers that

• works fast & reliable for many different folders to merge into the existing tree
• containing duplicate date folders and new date folders to the same time
• without re-importing metadate (which happens when syncing folders!)
• contains steps within OS (Win7/Win10 File Explorer) AND within Lightroom
• I add an explanation why I do as I do ...

Here is the Procedure:

1. I Process Year by Year Note: In principle all Merge can be done at once as well, but repeating it for every year is not to much of extra work
2. I call the Source Tree Source/YYYY/YYYYMMDD (my "New" Folder) the Destination Tree Destination/YYYY/YYYYMMDD (my "Fotos" folder)
3. File Explorer: Move & Merge trees
   • Move Folder YYYY from Source to Destination (e.g. 2020 from New to Fotos)
   • Explorer Asks (Pop-Up): Confirm Folder Replace: This Destination already contains a folder named 2020. Do you want to merge the folder ... with this one? -> YES
   • Exporer shows the Progess window "Merging ..."
4. Lightroom: Update Catalog WITHOUT losing / re-writing Metadata, changes, edits
   • in Navigator Window select Source/YYYY (e.g. New/2020)
   • since it has been moved away outside of LR, Folder and all Subfolders show "?"
   • Right-Click Source/YYYY (e.g. New/2020) and select "Find Missing Folders ..."
   • In the Pop-Up navigate to the merged folder Destination/YYYY (e.g. Fotos/2020)
   • Click Folder and confirm Button "Select Folder"
   • LR shows a confirmation Pop-Up "The selected folder or one of its Subfolders is already in Lightroom. Do you want to combine these Folders?
   • Confirm Button "Merge" and the magic starts to happen! The Source folder disappears from the catalogue The Merged folders and files appear in the Destination folder in the catalogue All catalogue data (metadata, edits, keywords, ...) are preserved Thousands of images merged in a single short procedure

I hope you find this summary helpful! Cheers Wolfgang (aka Spaceman71)