upgrading from 2.3.7 to 2.3.13 - reconstruct -r needed?

[Sam Smith]

When I upgrade from 2.3.7 to 2.3.13, on a 64 bit machine, do I need to 
do a reconstruct -r on the entire user tree?
I'm looking at the ugrading doc, and it says "Due to byte alignment 
issues in cyrus.index, all mailboxes will have to be reconstructed." 
when upgrading from 2.3.3 or later on 64 bit machines.

Will the user's email be accessible while that reconstruct is running? 
If not, that's going to be a significant downtime...

Thanks for any help,
Sam Smith