This is why they are doing it…
https://www.zen.co.uk/ip-leasing/

To be fair they have not offered the /29's as an included option for quite a few years.
I and I would guess a lot of others have them as its been a "passed down" option over several renewals.
How they went about it and their original response to quires was what got me.

To me changing allocations goes against the definition of static.
So they now doing this a second time, I think the questions you raised in your post are valid.

In reply to a post by devonkev:

This is why they are doing it…
https://www.zen.co.uk/ip-leasing/

Well not always. In fact it seems unlikely. For IP leasing they can lease from within their existing network, unless someone wants more than a /24 I am sure they have spare whole /24s for lease purposes.

I was in the first batch of IP re-assignments. On 82.70.0.0/16. They didn't use those for lease, they sold the entire block. If you look up anything in 82.70 now it shows as belonging to Oracle Sweden.

I'd bet they got another sweet offer for one or more /16s and are selling them.

In reply to a post by David_W2:

Zen has backed down in our case. We will be moving to a replacement /28 at no additional one-off or monthly costs.

My logs show Zen tearing down the PPPoE connection at 08:24:33 this morning, just a little later than the 08:19 they had notified. We are now running with our new /28 - fortunately, I had a suitably edited router configuration file ready to install, so our total downtime (including changing static allocations on local devices) was around six minutes.

I have just got some zone file edits to complete, then reconfigure reverse DNS.