Does Spark have all of its functionality regardless of what version of Wildfire or Openfire you have installed?
I think it should have all the standard xmpp functionalities. What exact features are you talking about? Or what version of Openfire do you want to use and why? (i presume older) I cant think of any feature that will not work with older server version now.
We are still using Wildfire 2.5.0
I see that you have started a couple of threads about upgrading some time ago. So, have you tried to upgrade? What is stopping you? You can make a backup of whole Wildfire dir and revert to it if something goes wrong (though it shouldnt). Openfire setup should handle server name change and upgrade Wildfire to Openfire. Or you can install fresh Openfire and then copy embedded-db and conf\wildfire.xml to it. You will need to change name prefixes of db files and wildfire.xml to openfire.xml.
As i said i cant think of any functionality Spark can be missing with 2.5.0 server. Probably in that version there is no File Transfer Proxy that Spark can use or some other services, but all common functionalities (messaging, group chat, etc.) should work. I’m not sure if server is taking a part in voice chat. But i think it’s peer-2-peer feature.