Mail server advice

[AliceWonder]

I've got a mail server currently running on CentOS 5

Other than it is beginning to run low on space and I had to disable the spam filter (ClamAV) a few months back because it was using up the servers memory, it's been running fine since CentOS 5 was the latest thing.

Postfix for my MTA, dovecot for IMAP.

I'd like to move it to a server running CentOS 7 and would like to know if there are any particular pains I need to look out for.

In the current CentOS 5 setup, each mail account has a user account on the machine - which allows for individual users to have .procmailrc files for filtering incoming mail. Is it possible to still allow them to have server side filters w/o actual unix user accounts? That would be preferable to me, but there must be some means of server side filters. I want to say it is currently using mbox but I'd hate to have that term wrong - it has been so long since I set it up.

Is there a straightforward way of migrating old user mail to the new system w/o just rsync of the individual users mail directories?

The old VM only had 512k of memory, I'm hoping 1024k will be enough to keep ClamAV happy for awhile, it really made a difference on spam. Only a handful of users.