Endless upgrade: mention relationship with #6876 and #9373.

wiki/src/blueprint/Endless_upgrades.mdwn

@@ -311,7 +311,11 @@ The IUK size is involved in at least four concerns:
   `config/chroot_local-includes/usr/local/bin/tails-upgrade-frontend-wrapper`)
   so the upgrade would fail. This is a regression for users with 2 GB
   memory: for all of 2.x and 3.x, all IUKs have been under 400 MB,
-  which would work fine with 2 GB of memory.
+  which would work fine with 2 GB of memory. If that's a blocker, then
+  we have to solve it by changing the format of the IUKs
+  ([[!tails_ticket 6876]]); we should coordinate this with other changes
+  that will break automated upgrades from Tails N to N+1, such as
+  Tails 4.0 and the migration to overlayfs ([[!tails_ticket 9373]]) .
 * Bandwidth needs of the RM. Uploading 10 GB of IUKs can be a pain for
   some of us, but that can easily be solved by making it possible to
   generate IUKs on lizard (and then compare them with the ones you