If it is no longer in the configuration as described in the type
certificate plus any STC 's and/or field approved modifications. So,
if your winglets are not approved via an STC or field approval (337
form) and are not listed in the type certificate then you are not
flying under a valid airwothiness certificate. There is the question
of major/minor modification/repair but I believe winglets would not
pass the test of being excluded from the "major" catagory.