In either case you will need to create a new, handwritten-style file with the same parts as your original.

1. One way is probably to set up a parallel score in "handwritten" and copy from the original score into the new one.
   
   
2. A second way, IIRC: Finale 25.3 will let you import an XML of your file into a new empty file containing the same parts and the XML import will take on the specifications of the new file. 

I have tried (a long time ago) actually adjusting the fonts of the original document (actually a copy thereof) and recall it was possible but was a multi-step process.

If the original contains Finale default fonts and you're converting to one or more of Finale's handwritten fonts, it will take less work.

As Adrian suggests, don't expect it to take none. Sometimes, you get lucky.