The situation with EKZ notes has been known for a while. It appears that EKY and FKA are also interesting prefixes. EKY appears to end at or before 7000000, and FKA does not start until 3000000.
The comparison between HBG and EKZ is interesting. It does not matter if the Macklem-Carney HBG notes are replacements (or just inserts) because they are scarce changeovers. Their value will be based on being rare and the average collector's desire to have a note from each signature series. EKZ is more like ALZ, the $20 prefix that has just over one-tenth of a normal printing. Neither of these prefixes is a changeover of any kind. The difference is that ALZ followed logically in printing patterns from ALY. EKY terminates early, and then you have EKZ notes in a high number range where the starting and end points are completely unknown. This more closely resembles the start of the Journey series in the $10 notes where FDY terminated early and all the FDZ notes are numbered over 9000000. However, FDZ is a changeover prefix, and the first 600,000 FDZ notes (signed Knight-Thiessen) are considered inserts.
Taking a look at the SNDB numbers, it would be easy to conclude that prefixes EKA-EKY have been largely released while EKZ and notes starting with FK* are still in release, meaning many are still in the vault waiting to reach circulation. HOWEVER, we know that EKZ existed before the release of the polymer $100 series because a member of this forum obtained one at the launch. For the longest time, it was the only EKZ note in the database and it took quite a while for any notes anywhere close to EKZ to reach circulation. So I don't buy the idea that there is a raft of EKZ notes sitting at the back of the vault waiting to be released. In fact, I am starting to think EKZ was printed before any other prefix. That would explain why there is such a huge offset in the numbering of this prefix.
Years of research into examining prefixes and numbering have shown strange patterns that pop up very occasionally. The similarities between FDY-FDZ and EKY-EKZ are striking. That would make a strong case for EKZ being a replacement prefix. However, FDZ notes are not rare and were confirmed as inserts. EKZ notes are quite rare at present. My gut is telling me that EKZ notes were printed exclusively for the Bank of Canada. I think that the Bank of Canada, after all these decades, still needs to have a supply of notes on hand to use for their own purposes, and it would make sense that they get these notes before any other notes are printed. In the 1950s, 60s and 70s, these notes appeared identical to asterisk note issues, but we learned the difference between true replacements and so-called "star make-up notes". The smaller asterisk note ranges tended to be the ones that the Bank of Canada received and used for their own replacement purposes. The printers did most of the quality control processes themselves, but since the Bank of Canada is the authority that issues money, they still need to check the products they are putting into circulation, so their needs are always going to be met first.
Naturally, every proposed answer raises more questions. For instance, why should the first notes printed and sent to the Bank of Canada be "distinct"? In other words, couldn't the printers have started at EKA and sent the first million or so of these notes to the Bank of Canada? Of course, I don't know the answer. But I think this process of planning ahead happens quite often, more than we think. In fact, I am starting to believe that $5 Macklem-Carney HBG notes are very much like EJZ notes in the sense that they were printed before HAM-HAZ and HBA-HBF and their destination was the Bank of Canada. Only problem here is that HAM-HAZ and HBA-HBF were either never printed or very few were printed and never released because of the signature changeover created by the departure of Mark Carney as governor. My gut tells me that because HBG notes were printed with Macklem-Carney signatures before any other prefix, this forced prefixes HAM-HAZ and HBA-HBF to be abandoned for possible use with the new Macklem-Poloz signatures because, obviously, it would seem quite weird to have a block of Macklem-Carney notes suddenly appear in HBG after the release of tens of millions of Macklem-Poloz notes starting at HAM. In fact, had this actually occurred, the Macklem-Carney HBG notes would be considered "good-overs".
Back to the subject of EKZ notes, it does not look like there will be a major release of this prefix. Yes, it is early, but we are already seeing subsequent prefixes in circulation (FKA-onward). This tells me EKZ is destined to be rare. As for collectability, unfortunately, EKZ is a $100 prefix, and the $100 polymer notes are proving to be very unpopular. No doubt, there are very few prefix collectors for the $100 notes. Moreover, the comparison to HBG notes and the huge popularity of HBG notes is going to overshadow the EKZ notes at least in the short term. I don't know if it is a rule of thumb, but sometimes, notes that are not popular when they are new could become very popular when they are vintage. So maybe it's worth holding on to EKZ notes for the long term.