Classic Plan - ST no longer offer this plan, so the only way for you to retain it is not to renew your contract aka don't use the line to buy a new phone.
Until a time when ST decides to really force Classic Plan users to change plans whether you want to renew your contract or not, that is to charge more regardless. Yes, I'm one of those who plan to go under the orange umbrella if/when it happens.
On a side note: I thought these telcos have very low/non-data plans? Of course, these cannot compare with Classic...
Anyway, a common practice among the telcos that I've been questioning is - why force us to pay for SIM card-size according to the new phone we buy??? Example, I'm using a phone that uses microSIM then buy an iPhone. Telco will force to buy new nanoSIM. I question this along the line of IDA rule that phones sold by these telcos are not allowed to be locked (telco-lock) - forcing us to buy the new SIM is like forcing us to use said new SIM with the new phone, indirectly locking it to said telco.