The answer is that, enlightenment is not something you can achieve. If it was, then it would be produced and conditioned and could be lost, as you say you could become more or less enlightened or de-enlightened.

“It is incorrect to think that Nirvāṇa is the natural result of the extinction of craving. Nirvāṇa is not the result of anything. If it would be a result, then it would be an effect produced by a cause. It would be saṃkhata ‘produced’ and ‘conditioned’. Nirvāṇa is neither cause nor effect. It is beyond cause and effect. Truth is not a result nor an effect. It is not produced like a mystic, spiritual, mental state, such as dhyāna or samādhi. TRUTH IS. NIRVĀṆA IS. The only thing you can do is to see it, to realize it. There is a path leading to the realization of Nirvāṇa. But Nirvāṇa is not the result of this path. You may get to the mountain along a path, but the mountain is not the result, not an effect of the path. You may see a light, but the light not the result of your eyesight.”

The Third Noble Truth - Walpola Rahula: What the Buddha Taught

“An Arahant after his death is often compared to a fire gone out when the supply of wood is over, or to the flame of a lamp gone out when the wick and oil are finished. Here it should be clearly and distinctly understood, without any confusion, that what is compared to a flame or a fire gone out is not Nirvāṇa, but the ‘being’ composed of the Five Aggregates who realized Nirvāṇa. This point has to be emphasized because many people, even some great scholars, have misunderstood and misinterpreted this smile as referring to Nirvāṇa. Nirvāṇa is never compared to a fire or a lamp gone out.”

It’s very hard to talk about enlightenment. You have to drop the idea of it as something you can possess or achieve as a state. But we don’t have any other way of thinking about things. If we could understand what it is to be enlightened we’d be enlightened already. Quoting from Walpola Rahula again:

“Nirvāṇa is beyond logic and reasoning (atakkāvacara). However much we may engage, often as a vain intellectual pastime, in highly speculative discussions regarding Nirvāṇa or Ultimate Truth or Reality, we shall never understand it that way. A child in the kindergarten should not quarrel about the theory of relativity. Instead, if he follows his studies patiently and diligently, one day he may understand it. Nirvāṇa is ‘to be realized by the wise within themselves’ (paccattaṃ veditabbo viññūhi). If we follow the Path patiently and with diligence, train and purify ourselves earnestly, and attain the necessary spiritual development, we may one day realize it within ourselves – without taxing ourselves with puzzling and high- sounding words.”

The different schools have many different ways of understanding enlightenment and talking about it. But they all share this, that it’s something you have to realize it for yourself.