How may I copy all existing "MEMO" entries from internal memory to the SD card on S7?

Maybe you can use Samsung Smart Switch, Samsung Kies, Coolmuster Mobile Transfer to help you transfer your memo app from your old Samsung to the new Samsung Galaxy S8+.

Samsung has done away with the "Memo" app and has changed it to "Samsung Notes". If you used Samsung Swith all saved memos from your S7 will transfer into the Notes app. Hope this helps.

Just transferred my Memo data from my S7 Edge to my S8

Use Smart Switch, select the option for Memo and they are transferred to Samsung Notes