Lifetime Guarentee Policy changed after purchase!

So I purchased a tempered glass cover for my iPhone 6s Plus which was fitted in store. Was told the cover came with a lifetime guarentee (fabulous!) Not even a month later I realized the glass cover had a huge crack in all the way down the screen, and I have never even dropped my phone! My husband was in the area and went to the Cavendish branch who said, yes it has a lifetime guarentee, all I need to do is bring it in for replacement. 2 weeks went by before I could get into the store and all of a sudden the policy had changed (2 weeks ago) and now you have to register the packaging warranty online etc etc, which obviously I never kept as this was not the policy when I bought it! They even admitted that the supplier had changed their policy. How can someone change a policy after you have already purchased?? I can understand purchasing a new product with the new policy, but surely this is not legal? I am very frustrated as these covers are not cheap! And I feel like I have been taken for a fool!