you can not change positive feedback
If that feedback and the other one you left is reported to ebay then they will remove it and you get a black mark on your account ( or two)
Sellers are not allowed to leave negatives for buyers and that is why there is no red dot.
It is against ebay policy to leave a positive dot with a negative comment no matter the reason
If you have a non payer then you wait 4 full days. Ebay send reminders and after 4 days either you or ebay can cancel the transaction and you get your fees back the non payer gets a non payment strike on their account.
That is much more effective than feedback
You put them on your blocked bidders list
Non payers will have problems buying off many sellers who set their buyer requirements properly.
https://www.ebay.co.uk/help/selling/listings/setting-buyer-requirements?id=4152
