Lightroom :: Date Time Original Has Been Overwritten?

I've been using LR3 for a year or so and have an imported folder with over 7000 images in it all from 2009.  The set was originally created in Elements organsier 8.0 probably in 2009?
 
I have noticed that somewhere along the line 658 of them have had the original capture time overwritten to a single point in time 15th Nov.
 
They are all JPEG (before I had a RAW camera) and the changes are embedded in the jpg.
 
This folder has been processed with a few tags and some editing and filtered out rejects.
 
I would like to get the original time stamp back as the images do not flow with the other 2000+ from the same couple of months.
 
I do have copies of the originals with correct time stamps - but no tags etc as they were created at time of import.
 
I have tried copying one of the originals from backup and overwriting the 'wrong date' file using windows and then importing the metadata from file and this rectifies the date - but loses keywords etc
 
If I use LR to import from back up - firstly it does not see the backup as a duplicate - despite file name being the same - I guess date throws it??  so I end up with duplicates - one on each date ; correct + incorrect
 
My question is: Is there a way to re-read the original time metadata back into the file/catalog and retain the keyword tags (like a selective merge)?
 
It would be great to have a solution rather than manually having to identify the incorrect files and copying,  Or doing a mass read of original metadata and losing any keywords.