You just need to apply for the upgrade, a technician will go to your house for testing, if the test success then he'll activate your account for 4Mbps and will automatically change to the new subscribed speed in around 3 days.
One of my company's client just upgrade the ADSL, a technician came to test if the signal is strong enough to support it, it was a success and immediately activate the account on the spot and ask me to speedtest again in 3 days, simple as that. You'll get a bill with some deduction of your current subscription (until the date of the activation of new subscription) then follow by new subscription amount (2 months), you'll get the bill after 2 months.

All of this information is after I phoned so many times, liaised with so many customer support to come to this conclusion, different person with different method but it came to the same thing.