Hello @Praveen_Agarwal
In my opinion, since the land was purchased by the partnership firm and recorded under Long Term Investment in the ITR filings (since 2020), it is likely to be treated as a capital asset rather than stock-in-trade and it will be taxable as LTCG.
Additionally, since the sale of land is already covered under your existing business objectives, you may not need to amend the purpose of the partnership deed.
However, if the Income tax department argues that the firm is in the business of trading real estate, then they may classify the sale as business income instead of capital gains. To strengthen your claim of LTCG for this sale of land, you should maintain records proving that the land was always held as an investment asset (not stock-in-trade) and that the firm does not carried out frequent land transactions.