Upgrade of SDK versions

If I purchase the official edition of Smart Life App SDK and upgrade to v5.x, will the normal running of the app developed based on v3.x be affected?

If I keep using v3.x to develop and launch the app, do I need to replace the values of AppKey and AppSecret with those for v5.x?

The legacy OEM app and SDK-based app share the same Tuya account system. If the SDK-based app upgrades to v4.0, will the account system be affected?

After I upgraded to v5.0 or later, an error has occurred, saying that “SING_VALIDATE_FALEDerror: Permission verification failed”. How can I fix the problem?

The Tuya IoT Development Platform prompted me that SDK v3.x cannot be downloaded and asked me to subscribe to the official edition and upgrade to v5.0 or later as soon as possible. What is the latest time allowed for me to upgrade the SDK?