Hi, Monique,
The History function is a read-only function which is why no changes can be made there.
However, if you Copy the quote from History back to Quotes, you'll be able to change them from a Prospect to a Customer at that time. You'll probably want to delete the original quote from History also.
After, you're able to archive or copy the same quote to History again.
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Regards,
Softrak Tech Support