hi, do u mean  #hotlink  ? 
Don't believe them.. nowadays, those CS only try to con u or cheat u to change your existing plan to newer useless one.
If u change your plan, u cannot get your old plan back anymore.
The #hotlink that we knew about ,doesn't support validity365,..

U can simply ask them to disable the GPRS for your old plan, then u won't get GPRS charge anymore...
If they refuse to do so, u can simply complaint to mcmc/skmm . as simple as that..   
and  of course, after disable gprs, your friends don't have mobile data anymore..(don't worry, u still can use the WIFI)
but, they don't have to change anything else or change the old habit..
and don't need other un-clear term-and-condition "suspiciuos" plan ...
In case they still need a mobile data after that, simply asking them to get a dual sim phone, and grab another sim card from other telco, just for that mobile data.
(Hotlink simply don't want your money from the mobile data, so they create alot of hassle to u. so u don't buy data from them, just buy mobile data from others telco.. )...

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p/s: in case u don't know, modern android phone had ability to switch to mobile data while Wi-Fi is weak(mostly under WIFI advanced option)..
u have to make sure it is DISABLED, or u got PPU charge .... If u don't want to disable the gprs completely from the holink old plan...

btw, even if u do everything right, u might still get the shocking charge  ...their system has flaw,might charge u illegally, even u disable mobile data from the phone, every setting is correct, and use wifi only.
but, u can simply complaint  to mcmc/skmm, and claim your money back,  if u face this issue..
(the issue seem make a huge come back within the post pkpb era)...