Do you already have a subscription or Pay&Go card? In that case you can easily switch to another rate plan free of charge.
In MyProximus, only one member of the family (the "administrator") can manage all the subscriptions (Internet, mobile, TV, fixed line, etc.) with the same customer number. The rest (the "users") only see their own usage. Are you an administrator or user in MyProximus?
Did you choose a mobile phone when you took out a subscription? Usually, you can change your rate plan afterwards, but you can't change your DataPhone option.