yes thars true during mahabharat bheeshm coverted shakuni's father caste(vaishya)to a khatriya on some bet about a horse
a. Aitareya Rishi was son of a Daasa or criminal but became a Brahmin of highest order and wrote Aitareya Brahman and Aitareyopanishad. Aitareya Brahman is considered critical to understand Rigveda.
b. Ailush Rishi was son of a Daasi, gambler and of low character. However he researched on Rigveda and made several discoveries. Not only was he invited by Rishis but also made an Acharya. (Aitareya Brahman 2.19)
c. Satyakaam Jaabaal was son of a prostitute but became a Brahmin.
d. Prishadh was son of King Daksha but became a Shudra. Further he did Tapasya to achieve salvation after repenting.
(Vishnu Puran 4.1.14)
Had Tapasya been banned for Shudra as per the fake story from Uttar Ramayan, how could Prishadh do so?
e. Nabhag, soon of King Nedishtha became Vaishya. Many of his sons again became Kshatriya. (Vishnu Puran 4.1.13)
f. Dhrist was son of Nabhag (Vaishya) but became Brahmin and his son became Kshatriya (VP 4.2.2)
g. Further in his generation, some became Brahmin again (VP 9.2.23)
h. As per Bhagvat, Agniveshya became Brahmin though born to a king.
i. Rathotar born in Kshatriya family became a Brahmin as per Vishnu Puran and Bhagvat.
j. Haarit became Brahmin though born to Kshatriya (VP 4.3.5)
k. Shaunak became Brahmin though born in Kshatriya family. (VP 4.8.1). In fact, as per Vayu Puran, Vishnu Puran and Harivansh Puran, sons of Shaunak Rishi belonged to all four Varnas.
Similar examples exist of Gritsamad, Veethavya and Vritsamati.
l. Matanga was son of Chandal but became a Brahmin.
m. Raavan was born from Pulatsya Rishi but became a Rakshas.
n. Pravriddha was son of Raghu King but became a Rakshas.
o. Trishanku was a king but became a Chandal
p. Sons of Vishwamitra became Shudra. Vishwamitra himself was a Kshatriya who later became a Brahmin.
q. Vidur was son of a servant but became a Brahmin and minister of Hastinapur empire.