Is there praying and fasting in Heaven? Once you are in Heaven you are assigned a "grade" level could you go higher than that level by performing certain duties?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Paradise - May Allah make us and you among its dwellers - is a reward to the good pious people and it is not a house of assignments and trials. Ibn Al Qayyim stated that all worship will stop in Paradise except the Dhikr (remembrance of Allah) which is permanent.
Muslim reported from Jabir that the Prophet said: I heard Allah's Apostle (peace be upon him) as saying that the inmates of Paradise would eat and drink but would neither spit, nor pass water, nor void excrement, nor suffer catarrh. It was said: Then, what would happen with food? Thereupon he said: They would belch and sweat (and it would be over with their food), and their sweat would be that of musk and they would glorify and praise Allah as easily as you breathe.
So, there is no fasting nor is there praying in Paradise but only Dhikr (Tasbeeh, Takbeer, Tahmeed, Tahleel) and with moderation. In fact, there is no kind of hardship in Paradise.
The deeds of the person in this life might make him deserve a certain status in Paradise. But Allah elevates that person and gives him a higher status sometimes out of His Mercy or because of the mediation of one of those whose mediations are accepted by Allah or just to make him join his good pious parents or even because of his love to the good pious people. Allah Says (interpretation of meaning): {And those who believe and whose offspring follow them in Faith, to them shall We join their offspring, and We shall not decrease the reward of their deeds in anything. …….}[52:21].
A man once asked the Prophet saying: "O! Prophet of Allah what do you say about a man who likes some people but can not reach their level?" The Prophet (Peace and blessing be upon him) said: "The person is with those whom he likes". [Reported by Imams Bukhari and Muslim]
Allah knows best.