My father-in-law has run into exactly this problem - documents he started working on in an earlier version of Word with Adobe Caslon are now no longer editable or printable because the new Caslon doesn't have the private codepage ligatures that he used. He's working on transcribing original prints from more than 100 years ago which used ligatures throughout.

It's very frustrating to have to go back and fix things that were done the correct idiomatic way at the time.