pg_checksums?

Hi,

I have a few questions about pg_checksums.

Long story short, checksums were disabled in our environment but are getting enabled now (again!) because of block
corruptionswhich destroyed a few databases in a database cluster. And before you say "told you so", the decision to
disablechecksums was not mine. Some people just have to learn the hard way. 

Anyway, re-enabling data checksums creates a few questions:

a) why isn't it possible to enable checksumming while a database cluster is up?

b) why isn't it possible to check whether checksums are enabled or not?

c) in a Patroni cluster consisting of a primary and a sync standby, is it safe to enable checksumming in the replica,
thenswitch over and enable it in the ex-primary, i.e. now new replica, without any penalty? Or do I have to perform a
reinitto really get them in sync again, though paronictl happily reports them to be in sync? 

d) how long does it take to enable checksums in general? Minimizing down-time is crucial. Does it depend on the
databasecluster size, or the number of files it uses, or what can be taken as a criterion to estimate then necessary
down-time.

Thanks in advance for your insights.

Cheers
Paul