Hab ich gerade gemacht, ging problemlos.

Attention important, if you have paid via credit card until now, you have to stop it otherwise there will be a hassle. In the end, however, it was necessary to make a few phone calls to support.

Just renewed. It worked. It now says next debit 12 December 2021 instead of 12 December 2020. Thank you Digitec for the great Black Friday offer.

I had the same question, as my subscription does not expire until the end of August.
At https://helpx.adobe.com/download-i... it says the following:
If your payment plan is Annual plan, prepaid, the redemption code term is added to the end of the paid subscription term.

Now I ordered the code from Digitec and redeemed it on my Adobe account.
I had to wait a few minutes for it to update.
Now it says my plan doesn't expire until August 2021.

Now, of course, the question is whether I could extend it again until August 2022. In theory, I would say "YES". But that wouldn't be fair in my opinion, so I'll let it go now and be happy with what I have now.

Adobe has a FAQ entry on voucher codes. You can find it at
https://helpx.adobe.com/ch_de...

Attention, note the section at the bottom:

"If you purchased your subscription from a reseller.
You can purchase your subscription from any reseller who sells Adobe products using coupon codes. In this case, your redemption will work the same as the subscription purchased from the Adobe website, unless your payment plan is Annual Plan, which is paid monthly. This plan is not sold through the reseller channel."

Status as of 2023.

It does not work if you have an active but cancelled plan. The old and the new prepaid plan become active at the same time.

No problem, enter the code, wait a few minutes until the confirmation email arrives and the subscription is renewed (date updated on Adobe page).

It also worked for me without any problems
- logged in with my adobe id
- called creative.adobe.com/redeem in second browser tab
- entered the code
Didn't take 30 seconds until the expiry date was updated (mine now says Next payment due on 28 Oct. 2022)

I have also just redeemed it - annual subscription expires in November 2020 - renewal according to Adobe account until November 2021. It was quick and uncomplicated.