There is no easy way to change cards to another type. The main way is to add them as customers and then change the sales receipts to reflect the customer name instead of the Other card.
Another way is to export your Other Names (File > Utilities > Export > Lists to IIF Files), then open the file in Excel, and copy/paste the information into Lists > Add/Edit Multiple Lists Entries.
However, the names can’t be exactly the same, so you could change the ones in Other Names to make them slightly different eg adding a number, symbol or letter. Then once imported back, inactivate all the Other Names so you don’t accidentally use them again.