upgrading cluster with 2 nodes to new major version

Hey,
I have an environment with 2 nodes (one PRIMARY, one secondary in read
only) that has pg 9.6 version with repmr 4.3.
I'm trying to upgrade both nodes to pg11 according to the following doc :
https://www.postgresql.org/docs/current/pgupgrade.html

I wanted to get your opinion regarding the rsync command. According to the
docs, I need to run the following rsync command in the PRIMARY for each
secondary node :

rsync --archive --delete --hard-links --size-only --no-inc-recursive
primary_old_cluster_data_dir primary_new_cluster_data_dir
secondary_remote_data_dir

I wanted to run the following rsync command from the secondary :

rsync --archive --hard-links --size-only --verbose --human-readable
--no-inc-recursive --delete root(at)primaryIP:PRIMARY_OLD_PGSQL_PATH
SECONDARY_PGSQL_PATH

However,
as it seems, when I run the first command from the primary for every
seconday the rsync succeeds and I can start the db without any issues.
On the other hand, after running the second command from the secondary I
fail to start the db and I get the following error :
The database cluster was initialized with PG_CONTROL_VERSION 960, but the
server was compiled with PG_CONTROL_VERSION 1100.

Can anyone explain why is it mandatory to run the first command on the
primary ? and why cant I run rsync from the new dir in the primary to the
old dir in the secondary ,why I need to specify 3 directories according to
the docs ?

Last question - after the upgrade (when I use the first command) my
replication slots are deleted. any idea why ? Is it normal after upgrade
and I should create them from scratch ?

thanks.