I am advised by my colleagues to change my plan to iValue Simple as I intend to get a pre-owned iPhone 5S from him.
Do I need to walk into Maxis Center at KLCC or Sunway to do this or just via phone call to 123 will do?

Dude, I don't think its possible to subscribe to the iValue plans. Last time I bought an iPhone 5 from a LowYat Plaza and thought to apply to iValue 1. When I called the CS to change the plan, they told me that I need to purchase the device & plan together with Maxis in order to subscribe to iValue 1. If bought outside, I can only choose the normal plans like surfmore or talkmore that time, but you can always counter check with Maxis CS just to reconfirm as well.

Maxis, please correct me if I'm wrong!

I checked their website, really lacking info. I thought get the phone from Maxis only got contract. Why now no need get phone also cannot subscribe to a plan?

As i know, I-value plan it comes together with the device hence its not available if you just want the plan only.
try to check their other plan la - http://www.maxis.com.my/en/personal/postpaid-plans.html