If you accept their 'like for like' suggestion the warranty will transfer. If you upgrade (even if you put so much as a penny towards a new set) it is classed as a new sale with a new warranty.Can you provide any links to back that up please? I can't find anything to substantiate that one any of JL's T&Cs, but there are plenty of sites talking about JL warranties that basically say the same as this quote from Money Saving Expert:
"Under John Lewis's guarantee, it will fix or replace an item providing there is no evidence of misuse or accident.
If your product is replaced within the guarantee period, the guarantee isn't renewed, but instead continues to apply from the date you bought the original item."
I can't believe any retailer would operate a scheme that restarted a warranty from the top if they had to replace an item after a claim.