I've had a look. Seems you'll have to get your hands quite dirty.
First step would be to leave your current installation intact

Then perhaps make a "newhorde" folder at the same level as your current horde folder.
Already it gets ugly - you need to tell the httpd.conf about this new location.
Once you have done that, go to
http://ftp.horde.org/pub/snaps/latest/ and get the components you want to play with (horde, turba, imp, kronolith, nag, mnemo, etc.) then untar them into your new horde2 directory - start reading the docs/INSTALL for each package - starting with the horde framework itself.
This will be bleeding edge stuff, and once you invest time going down this route, you can probably kiss goodbye to the RPM releases from Dan Brown et. al.
Well, maybe this gives you a push in the right direction?
See, I just tried to setup Jonah from the nightly snaps - it seems the horde framework that it's looking to use is sufficiently more recent than the version I have installed - it won't play ball. I am not sufficiently motivated to consider updgrading the whole horde framework (and then I guess IMP, kronolith, etc, etc, as well.) so I quit.
G