Why do you need to be the account holder all of the sudden to swap YOUR own phone? It doesn't make any sense. I know I have done it before in the past when I wasn't an account holder. It's my phone and its defective so why can't I just swap it out through the total equipment protection plan or what ever its called now.
Because while YOU may have paid for the phone, YOU are not responsible for the account and/or payments/changes to the account. The person who actually signs the contract is, i.e. the account owner, and unless this person gives you authority by making you an account administrator you will not be able to do so.