standby upgrade questions

From: https://www.postgresql.org/docs/9.6/static/pgupgrade.html

Q1 - Is there a procedure to force this "catch up?"   I suppose
preventing db activity with a pg_hba hack and running "select
pg_switch_xlog()."   Maybe there's a better way?

1.

Prepare for standby server upgrades

If you are upgrading standby servers (as outlined in sectionstep 10
<https://www.postgresql.org/docs/9.6/static/pgupgrade.html#PGUPGRADE-STEP-REPLICAS>),
verify that the old standby servers are caught up by
runningpg_controldataagainst the old primary and standby clusters.
Verify that the"Latest checkpoint location"values match in all
clusters. (There will be a mismatch if old standby servers were shut
down before the old primary.)

Also, if upgrading standby servers, changewal_leveltoreplicain
thepostgresql.conffile on the new master cluster.

Q2 - I was wondering what the end result is, in general, of this rsync
of old and new? If the old db files and upgraded, new db files are
meshed what do you get on the other side of the rsync?  Again, I mean in
general what is the goal?

f. Runrsync

From a directory that is above the old and new database cluster
directories, run this for each slave:

rsync --archive --delete --hard-links --size-only old_pgdata new_pgdata remote_dir

whereold_pgdataandnew_pgdataare relative to the current directory,
andremote_dirisabovethe old and new cluster directories on the standby
server. The old and new relative cluster paths must match on the master
and standby server.