Welcome to Lacerte Letter Frustration - you are not alone. 

Has either of these clients been submitted for e-File? If so when? Clients I have completed e-File for have one date, and clients waiting for submission will change based on Update. Completed client letter due dates will not change - at least not in my experience.

Once IRS stops spinning Wheel... Of... Due-Dates! Maybe @IntuitAustin and Lacerte will actually be able to catch-up...

Or in Forms, while viewing the Letter, Right-Click and use Jump To Input and Edit the Letter yourself to avoid pesky waiting. Careful it's a finicky Editor. (Not Organizer Questionnaire finicky but still watch your step.)

Oh and I would not make changes to the existing Letter when it asks you to Save Changes just print/PDF what you need and let the Default remain.

No matter what you do. Client will either completely ignore it, or hound you with impassioned cries for yet more hand-holding. Godspeed Preparer!