In Jainism, a bhavya jīv can never become abhavya, and an abhavya can never become bhavya.

Reason in simple words:

• Bhavya jīv = soul that has the capacity for moksha (liberation).
• Abhavya jīv = soul that does not have the capacity for moksha at all.

This capacity / incapacity for liberation is called bhavyatva and abhavyatva, and Jain āgams and Tattvārth Sūtra describe them as permanent modes (parināmika bhāv) of the soul, not something that changes due to karmas or effort. So:

• A bhavya may wander in saṃsār for infinite time, do severe sins, fall spiritually, etc. – still it remains bhavya.
• An abhavya may do puṇya (merit), get good births, enjoy heavenly pleasures – still it can never attain samyak darshan and moksha, so it remains abhavya.

Important point for us practically: We can’t know for any particular jīv whether it is bhavya or abhavya, and Jainācāryas say we should always assume ourselves to be bhavya and put full effort in right faith, right knowledge and right conduct.

You can read a bit more about the basic idea of bhavya and abhavya here