I needed something to do.
Selected a photo direct from a camera and one from an iPhone. Created a test Photos library and imported both originals.
Exported using the standard Export command (with both “Title,Keywords, and Description” and “Location” checked, exported with Export Original command, and located the “original” from the Photos package via the Finder (file called Masters in older versions of MacOS, now called Originals).
Used an online EXIF display tool at exifinfo.org to display the information from each of these photos. What I found was the original photo data agreed with the data from the Master/Original file from Photos, and the files exported using the Export Original command. The data from the normal Export command was not exactly the same, and in some cases had more data, and some cases. less data.
Suggest you try a similar test and see if you come up with a solution to your problem.
From an easiest solution point of view, using the Master/Original files as a source to import your older library to the new seems the best route to me if you are happy with the results of the exif testing.