I faced the same problem - sent the following email:

Dear Maxis
Rcvd your letter dated 18th Oct regarding plan enhancement and read it with little interest. As the plan in focus is currently used by my 80-yr old dad who basically just needs a 'mobile telephone', the offer was not attractive.
Since your letter did state that "if we do not hear from you by 15 Nov 2016, we will proceed to update your rate plan...", I called you customer service to inform that the offer is great but no thanks.
But to my utter disappointment, you cs rep (Ms.Tini) said that my existing plan would be made redundant and that I will be upgraded anyway.So why give me the choice???

How is this fair marketing and product enhancement??? I am appallled that you treat customers this way - although we have quite a monthly commitment with you (for all my three lines). It is extremely unfortunate and I think you need to re-think who drafts such letters!! I didnt have to bother calling your #123 and only to wait for a call back at 2308.

Unpleasant experience
sudha

it is very unfortunate tht such big corps behave this way. I have no choice but to say id' terminate the line but doubt tht even makes a dent in their already bloated bottom line. Anyway am glad to know tht what I am feeling is legit and valid.