An in place upgrade from one passenger version to the next works fine, but
the enterprise documentation mentions that you should uninstall the open
source gem before installing the enterprise version. Does this mean one
cannot do an in-place upgrade from open source to the enterprise version?
Or is the module compiled in a place that's not affected by the uninstall
so apache keeps running?

Phusion Passenger's gem is called 'passenger' while Phusion Passenger
Enterprise's gem is called 'passenger-enterprise-server'. However the
names of the binaries (passenger-status, passenger-memory-stats)
remain the same. If you install 'passenger-enterprise-server' when
RubyGems will not automatically remove 'passenger'. The
'passenger-status' and 'passenger-memory-stats' wrapper scripts
generated by RubyGems could get confused as to which gem to activate.

And this is pretty much the only reason why we recommend uninstalling
first. There are no other conflicts.

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Thanks for the feedback. With "in place" upgrade I meant to ask whether
it's necessary to take the server offline during the upgrade, or whether
you can simply keep it running during the upgrade (apart from restarting it
after the config is updated, of course)
-j