I understand that this might be the best long term approach. What are the impacts of not upgrading? As I read the link in the question - the changes are not mandatory if users require company specific logins.
"Sign in with Apple is not required if:
Your app exclusively uses your company’s own account setup and sign-in systems."
What would the impact of not upgrading our builds to 8.3 mean - Our apps would stop working? Or we wouldn't be able to upload new versions of it going forward?
Honestly, ROPC grant is the wrong authentication for an end-user client application. OIDC with authorization code grant is the recommended best practice by NIST and other organizations for securing client applications in the wild. Apple guidelines are targeting apps distributed through the App Store and who are not exclusively using their own organization-managed authentication. If you fall into this category, you must move to Pega Infinity Mobile Client by April 2020, or Pega will not be able to support your app in the Apple App Store.
While you don't need to worry about the Apple ID login requirement you still need to upgrade to Pega Infinity. Otherwise you'll not be able to publish any update to your app store application after March 2020.