Hello, as there is a lot of cry on support forum I decided to make this short tutorial.
1) Login to your account on tibia.com:
2) Click on Vouchers:

3) Adjust your premium time and Double EXP/Skill time, by using slider:

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4) Press "Adjust Durations" button:

This message should appear:

5) Active voucher:

Follow instructions (I didn't not active yet, so I don't know what message appear)
AFTER YOU ACTIVE 1 VOUCHER YOU CAN NOT ADJUST TIME AGAIN!!!
Code:
As long as there is no voucher activated, you can convert Premium Time Vouchers in other vouchers and vice versa by using the slider below, then clicking on the "Adjust Duration" button to confirm your choice
Hope noone will make mistakes like my friends on ventrilo.