I think that after the release of AYZ and/or EYZ, we might possibly see AZ* and/or EZ* prefixes, since denominational letter "Z" has never been issued in any three-letter prefixes on any denominations EVER! And if the $20 note (Birds or Journey) is to release denominational letter "Z" anytime, it may very well be the final new denominational letter in Bank Of Canada's history of three-letter prefixes.
So far, the $20 note has released five denominational letters (that being I, S, V, W, and Y) - only the demised $1 notes have had that many denominational letters in three-letter prefixes (A, C, F, L, and M - "M" is now a secondary $50 denominational letter). The demised $2 prefixes have had four denominational letters in 3-letter prefixes - B, G, R, and U. The $5's have denominational letters N, O, and P. The $10's have letters D, E, and T. The $50's have letters H and now M. The $100's have letters J and now K - "K" was another formerly retired prefix, originally on the demised $1,000 note.
I think that the $20 prefixes will have AZ* and EZ* before we go back to denominational letter "I" with the BI* or FI* prefixes. Now whether it will be on the Birds $20's or the Journey $20's, only time will tell.
And that's my twenty buck's worth.
Later, Jonathan