I did upgrade my 6 months super subscription to 1 year premium last year.
14-Nov-22, I have activated the 6 months super subscription via TP coupon.
Then on 6-Dec-22, I have upgraded to 1 year Premium.
As you can see the difference amount was 707.21 including taxes. The existing 6 months super subscription was canceled and the 1 year premium was activated.
So it should be possible to upgrade to premium from the 1 year super subscription too, by paying the difference amount.