There is neither a legal nor an ethical reason for changing the feedback.
Actually, accepting money to do this would be accepting a bribe.
Feedback gives your honest reaction to the transaction.
Because you were able to leave feedback, should we take it that you did not use the Resolution Centre?
Along with learning to read feedback (not just the percentage) for problems*, you should know that eBay and Paypal both have strong Buyer Protection programs.
The Resolution Centre is at the bottom of this page.
*Sometimes it is even obvious that the problem was with the buyer not the seller.