That’s a bugger.
Inside six months it’s their responsibility to disprove an inherent fault. After that, it’s down to you to prove there was.
You’re now into the realms of asserting that the goods should last a reasonable amount of time, and that could be up to six years. For a ten bob pair of specs you’re screwed, but for the price of a pair of Oakleys I’d argue that they’ve failed in an unreasonable amount of time. It’s hard to see why they think you’d suddenly be using “excessive force” after changing user-changeable lenses for half a year without issue unless there was some fault necessitating that.
Your contract is with the vendor and it’s their legal responsibility to make good. However, if they’re being dicks you might have better luck dealing with Oakley directly. Just bear in mind you don’t *have* to.