Yeah, found it:
"The actual number is that you have to pay original price to upgrade.
For example, If you purchased 1 year plus for 25%off, which is $45.
To update to extra plan, you need to pay 100 - 45 = 55$, not 100-60=40$
A real example explained by the one who mentioned this:
1070 days left, 3593 NT$ needed to upgrade to extra plan.
He explained that he always purchase annually subscription for 761 NT$ (1188$ originally)
So that his 1070 days worth 1070/365days*761=2230NT$
An extra plan annually cost 1988 NT$.
1070 days for extra plan need 1070/365*1988=5827 NT$
5827(total price)-2230(he already paid)=3597NT$
Which is pretty close to the system tell (3593$)"
Didn't expect for them to ask the difference for a discount you had take advantage of when bought the Plus, that's freaking mental... it totally disrupt the concept of discount.
People bought the Plus since it was discounted but now they have to pay the difference for a past discount to make an upgrade?
JFC if Sony if going all in this gen...