[Mimedefang] Upgrading from old version (2.28)

[mimedefang]

I'm running Mimedefang 2.28 and SpamAssassin 2.43 under RedHat 8 and would
like to upgrade to Mimedefang 2.45 and SpamAssassin 3.0.  The problem is
that I need to do this on an in-use production server and can't have
downtime more than a couple minutes and no lost mails.  I also have
somewhat of a customized Mimedefang script although I could probably just
start again from the default.

Am I just running versions too old to do a smooth upgrade?  What upgrade
process do I need to follow?  Any pointers to a detailed FAQ or other
guide would be welcome.

As a side note, is this a case where "if its not broken don't fix it"
shouldn't apply?  Do most users upgrade to each new release?

Thanks,
-Joel