It's a bit confusing in your comments in both threads whether/when you reported the 1099-R for the $6490 and what is on line 8 of your 8606's for 2023 and 2024 and as you're not mentioning that line; here's the other thread for ref...
https://ttlc.intuit.com/community/taxes/discussion/re-roth-ira-conversion-got-taxed-again/01/3797137...
refer to the 1099-R (distribution/conversion) and 5498 (contribution) forms which should confirm for you in which tax year to report which piece.
in the other thread you list 2023 8606 Line 13 as $6490 which implies you may have entered the conversion for 2023 without the 1099-R, tho don't mention line 8. You should end up with $6490 on Line 14 and that will carry over to 2024 8606 Line 2.
I'm not a CPA but I think if you don't have any changes on 1040 then you don't need to amend the return and can just send in the 8606 to record the correct basis; but for 2023 it depends what you reported on 1040 Lines 4a/4b and 8606 Line 8.
