Any easy way to search for & replace characters in JPG file names in a folder?
Finder is not the tool for this job, but I've always done this sort of thing via Terminal. 

To be safe, you should have a backup of your Mac since the mv command will rename everything it finds, and if more files match the "642*" pattern, they will get changed as well.

Here is a snippet of code to get to the folder, find the files starting with 642 and do the edit one by one in an automated fashion. (Obviously changing the path to the real location of your images):

cd "/path/to/your/images"
for i in 642*; do
    mv "$i" "${i/642/699}"
done


This will move all images matching the wildcard 642* (meaning it starts with 642 and the rest of the name can be anything), and moves them in order to rename the 642 to 699. It will overwrite any files that already have the destination file name, so be sure that they are files you don't want.
This assumes that you don't organize your images into subfolders too; change 642* to */642* if this isn't the case.