There is no way to do this directly in KiCad V5.1.x.
In KiCad-nightly V5.99 there is a “Paste special” in the popup menu that preserves the Refdes, but this is not implemented in KiCad V5.1.x.
The easiest way to restore the annotation probably is:
- Make a backup, just to add some redundancy.
- Print the schematic to a .pdf (or .svg) file for reference.
- Add a hierarchical sheet, cut the part of the schematic, paste in that sheet.
- Use the .pdf to restore the annotation.
Eeschema / Tools / Update PCB from Schematic with: Match Method: Re-associate footprints by reference
Step 5. should re-sync the timestamp values that KiCad normally uses to sync the schematic with the PCB. Once that is done, you can optionally re-annotate your sub-page, with for example starting from number 100.