Mahabharata 12.181.14

इत्येतैः कर्मभिर्व्यस्ता द्विजा वर्णान्तरं गताःधर्मो यज्ञक्रिया चैषां नित्यं न प्रतिषिध्यते

ityetaiḥ karmabhirvyastā dvijā varṇāntaraṃ gatāḥdharmo yajñakriyā caiṣāṃ nityaṃ na pratiṣidhyate

Our plain-language meaning of this verse is waiting for scholar review.