I upgraded to Catalina, and now my MS Office apps don't work.

Either upgrade to Microsoft 365 (subscription plan), single-purchase Office 2019 for Mac, or use the free, 64-bit LibreOffice Suite, which is a superset of MS Office, opens MS documents back to DOS, and a deliberate clone that has been in continuous development by one name, or another since its StarOffice introduction in 1992.

Upgrade to a current version, that's all you have to do.

Office 2019 or Office 365 work with Catalina.