It is said that before a being achieves buddhahood, he has to complete ten bodhisattva grounds. These are known as 'causal grounds'.
Upon seeing the truth ( path of seeing), a bodhisattva needs to meditate and abandon intellectually- formed delusions ,self grasping and overcome innate self-grasping.
There are 9 levels of innate self grasping : big-big, middling-big, small-big, big-middling, middling-middling, small-middling, big-small, middling-small and small-small.
Bodhisattvas on the path of seeing enter meditative equipoise on emptiness to overcome big-big innate self grasping.When their wisdom of meditative equipoise becomes powerful enough to act as the direct antidote to big-big innate self grasping, it has transformed into the wisdom of meditative equipoise of the uninterrupted path of meditation of the first ground. At this point the Bodhisattva advances to the Mahayana path of meditation to abandon the big-big innate self-grasping. When this abandonment has occurred, the Bodhisattva has attained the released path of the second Bodhisattva ground, Stainless. The good qualities of a Bodhisattva on this ground is multiplied a thousand times. These Bodhisattvas then arise from meditation and work to help others. Later, they enter meditation again to abandon middling-big innate self-grasping.
When Bodhisattvas have abandoned middling-big innate self-grasping they advance to the third ground. The process of entering into meditation and work to help others in their meditation breaks continue till they advance to the eighth ground.
On the eighth ground Bodhisattvas have abandoned all self grasping and all delusions, but they still have obstructions to omniscience. Once again, they enter meditation on emptiness. When they have abandoned the gross obstructions to omniscience they advance to the ninth ground to abandon the subtle obstruction to omniscience.
When Bodhisattvas on the tenth ground enter meditation equipoise on emptiness their wisdom becomes powerful enough to act as the direct antidote to the very subtle obstructions to omniscience. This concentration is known as the 'vajra-like concentration of the path of meditation'. It is also called the 'exalted wisdom of the final continuum' because it is the last moment of the mind of a sentient being. In the next moment the Bodhisattva becomes a Buddha.
In the final session, Bodhisattvas free themselves from the very subtle obstructions to omniscience and attain the final released path - the Mahayana Path of No More Learning. When this path is attained, the Bodhisattva has become a Buddha and has gained the exalted omniscient wisdom. At this time the meditator attains the four bodies of a Buddha. Bodhisattvas on the tenth ground have many emanations, and all their emanations work for the benefit of all living beings.
So once a being has become a buddha, his buddhahood will not degenerate because this being has abandoned all innate self-grasping and the subtlest obstruction to omniscience.
I think after attaining the eighth ground, bodhichitta will not degenerate.
No, a Buddha will not degenerate and become a sentient being but he can emanate as an ordinary being to work and help others.