I get what you are talking about, but.. The latest Openfire version is 4.1.6. Spark versions are 2.7 and 2.8.
In your case i would install 3.8 on the new server, won't run it. Copy whole Openfire folder from your old server on top of the new installation. Try to run it, see if it works. Of course, you won't be able to login with Spark to it right away while your old server is running and your network is not adjusted to server change (DNS, etc.). Also, keep in mind that in 2008+ there is an UAC protection, which will interfere with Openfire if you are not running its launcher via Run as admin. Of course, if you use service, there is no problem with UAC. Then i would try to upgrade to 4.1.6 with the installer.
P.S. i'm not aware of any issues that could make Spark 2.8 not to run with older versions of Openfire. Unless you need some specific feature like Client Control plugin. So, it may be that you will still have problems with Spark 2.8 after the upgrade as they may not be related to Openfire version. What exact problems do you get?