Sorry for the confusion! The original official answer to this thread referred to a solution that was only possible on EEBA, the precursor to Goodbudget. You can still change your Envelope type in Goodbudget, but the process is different now.
You can now move your Envelopes from Monthly to Annual or vice versa (or between any types of Envelope) by dragging and dropping them in your Envelopes list. You can see instructions and a video guide on doing that in our Help Centre in this article.