Hi Bob,
for a long time they had indicated by corresponding reports that a Songbook+ full version, which, if I remember correctly, was bought more than 5 years ago, will become a demo version again (and then have to be bought again) if a "Major update" is installed. If you do NOT install another update after the said message appears, you can continue to use the currently installed version indefinitely.
After the mentioned message appeared, I simply took out a "Bronze Sponsor" subscription (via in-app purchase). As a result, 0.99 euros will be debited from my Apple account every month and I can continue to use SB+ as usual (including updates to the latest version).
Best regards,
Chris