The automotive anology is as valid as any.It points out a contract is a contract.
Fengtao can do what he is doing now & from a date such as May 8 in this case make the contract what he wants it to be.From that point on it is up to the buyer whether they want the product with the new contract.
Just because Fengtao is the seller doesn't mean he can alter the contract after the fact.
I haven't read the ECLU but if he had defined "Lifetime" in it as lifetime of the product then that would be the contract.A seller hould never leave this open to interpretation.Because with lifetime undefined in the contract the buyers interpretation is equally valid.Ultimately a court will have to make the decision.
To me a product with a lifetime guarantee is either to be properly repaired or replaced with a product that will properly do the job.
All a seller has to do is put in the contract & a lawyer could do the legalese better:The lifetime guarantee is for the lifetime of the product & this seller considers that X years.
I want to make another point.I bet Fengtao doesn't think the copyrights to the product run out with the "lifetime" .So another company could just take the name & make an exact copy & start selling it.