Any way to put 3.4.x forum into "Read Only" mode?

[AtariAge]

I will be updating my forum soon from 3.4.x to 4.4.1 (or more likely, 4.4.2), and during this process I will be switching to a new server. What I'd like to do is take the forums offline, and after I dump and copy the database, put the forums back online in a "read only" mode.  Once I have the upgrade complete on the new server, I'll be updating the DNS to point the site to the new server. This process is going to take several hours, and that's assuming there are no issues, so I'd like to at least allow people to continue to read the forums (with a big notice at the top explaining that the forum is being upgraded and that posting isn't currently allowed).  

Is there a way to make the forums read-only outside of changing all the group permissions so people cannot post? This would apply to blogs and gallery as well, but that's less important. Ideally it would be nice if I could prevent people from responding to PMs (or creating new conversations), but if I have to edit groups and permission settings, I could disable the messenger for everyone as well.  

Any SQL queries that might make this easier (so I make all forums read-only, gallery and blog permissions read-only, and disable the messenger for everyone) would be a big help. 

Thanks in advance for any insight!

[bfarber]

I'm afraid updating each group's permissions is the only way to accomplish this (at least elegantly - you could temporarily adjust the MySQL user's permissions so that updates/inserts/etc. can't be made by the user, but it would show a database error to the user when they attempt to post).

[AtariAge]

Thanks @bfarber, I'll probably just write some queries so I can quickly remove all the group permissions as well as restore them if the upgrade goes awry and I need to revert to the 3.4.7 forum.  I definitely don't want to go the route of removing the MySQL user's write permissions!