Online deal - changing the period!!

I did an online upgrade on one of my contracts. I am with Vodacom since 2002. The only deal available online was 36 months. I only heard now that there is a 24 month contract on the same phone. I received the phone about 6 days ago. Now I want to change the contract period to 24 months, and they told me I have to send back the phone, and they will send me another phone on the new contract. If the period of 24 months was available online, I would have choose it. I am happy with my phone, and don't want to send the phone back, only want to change the period. When I talked to the lady about it, she just put the phone down in my ear. I have 9 contracts with Vodacom and can not understand why I need to send back the phone. Can't the the paperwork just be changed?

Please Vodacom!