Manusmriti 10.65 is most quoted Verse to prove Varna system is not birth based and that varna changes.
The verse is – शूद्रो ब्राह्मणतामेति ब्राह्मणश्चैति शूद्रताम् । क्षत्रियाज् जातमेवं तु विद्याद् वैश्यात् तथैव च ।।
The Śūdra attains the position of the Brāhmaṇa and the Brāhmaṇa sinks to the position of the Śūdra; the same should be understood to be the case with the offspring of the Kṣatriya or of the vaiśya..
Now to counter this Verse people show 10.64 –
शूद्रायां ब्राह्मणाज् जातः श्रेयसा चेत् प्रजायते । अश्रेयान् श्रेयसीं जातिं गच्छत्या सप्तमाद् युगात् ।।
If the child born from a Śūdra woman to a Brāhmaṇa goes on being wedded to a superior person,—the inferior attains the superior caste, within the seventh generation
So does it mean Manusmriti 10.65 is talking about Varna change in 7 generations based on birth? Well no.
Problem is that this verse says Shudra attains position of Brahmin, It do not say 7th generation, also it talks about Kshatriyas and Vaisyas, Previous verse do not even mention them, so if you say 7 generation is carried from previous verse then it should apply to Vaisya and Kshatriya too, however this will contradict Manusmriti itself, A child born to Kshatriya Mother by Brahmin Father is Brahmin itself according to Manusmriti 10.28
यथा त्रयाणां वर्णानां द्वयोरात्माऽस्य जायते । आनन्तर्यात् स्वयोन्यां तु तथा बाह्येष्वपि क्रमात्।।
As to one his own Aatma is born from two out of the three nearest castes, as also from his own caste, even so is the order among those out of the outer circle.
According to this- A Brahmin gives Birth to Brahmin from Kshatriya and Vaisya, also a person born from Kshatriya father and Sudra mother is Kshatriya. Medhatithi in commentary on 10.64 says children from Kshatriya father and Sudra mother becomes Kshatriya in 5th generation which contradicts 10.28 which says children born to 2 nearest caste is of same caste (आत्मा) they are Dwijas and eligible for Upnayana(medhatithi’s commentary) . Same with a person born from Vaisya Father and Sudra mother.
A person born from Brahmin father and Kshatriya mother is already a Brahmin then how come Medhatithi says he becomes Brahmin in 3rd generation?
So it is not possible that 10.65 is a continuation of 10.64 in fact it is not,
And manusmriti 10.6 which says a person born to next lower Varna is equal to father’s Varna but slightly tainted, however if taken true then why 7 generation rule do not apply here?
But Gautama Dharma sutra says even to change Varna from next lower (like from Kshatriha to Brahmin) it will take 7 births, manusmriti says he is already brahmin, contradiction here too
Now if someone says that 10.28 is actually Interpolation and so the rule stands, if you say so then you need to discard Medhatithi’s commentary and still if you carry generation rule from previous verse it won’t apply to Kshatriya and Vaisya, also previous verse talks only about lower varna’s 7th generation becoming of higher Varna, how does higher varna become of Lower varna is not written anywhere?
These are all in contradiction, commentators like Medhatithi tried to counter these contradictions by twisting the meaning of verse for example in 10.28 he translates अस्य as ब्राह्मणस्य which is not correct because previous verses are talking about Chandalas etc how come “Of his” is translated as “Of Brahmin”?
Point to be noted that Manusmriti 10.63 is interpolated which is clear from the shloka itself it says “अब्रवीन्मनुः” which means “Manu said it” this indicates that this verse is not said by manu but by someone else in name of Manu.
It is clear that if you take 10.65 as a continuation of 10.64 then there will be many many contradictions with other verses Manusmriti itself. Also literally it means Brahmin becomes a sudra and Sudra becomes a Brahmin whereas 10.64 says 7th generation becomes Brahmin.
These parts are actually Interpolated in Dharma shastras in medieval times as they are inconsistent with other shastras, Fir example Gautama Dharma shastra too approves this 7 generation rule but it also says Sutas etc don’t have Upnayana Rights, however Mahabharata not only mentioned Sutas had Upnayana but also they were well educated in Vedas and smritis.
Hence the Birth based Varna system is a medieval time construct and it was reinforced by interpolation verses.
Bhavishya Puran Brahma Parva chapter 40
Even Bhavishya Puran uses the exact verse of Manusmriti 10.65 regarding changing Varna in this birth only and not in 7 generations, proving my point.
10.64 is not related to 10.65
Lets take up a case of this 7 generation rule, for example a Shudra woman marries a Brahmin male, their son will be a shudra only, now if that shudra male marries a Brahmin female, their son will be a Chandala, as it is now Pratiloma marriage, this 7 generation rule applies to only female not male, because if male will marry woman of higher Varna, it will be Pratiloma marriage.
However manusmriti says शूद्रो ब्राह्मणतामेति here शूद्रः is in masculine form, it should be शूद्रा because only women will attain position of brahmin in 7 generation.
As Manusmriti uses masculine form, it is clear that 7 generation rule is not applied in 10.65 at all