I have no idea how to modify an existing binary to use the 64 bit JRE but if you want to open a shell or batch/cmd file with the following content Openfire should start.
path-to-64-bit-java\bin\java -jar ../lib/startup.jar
This should print out that Openfire is starting fine and make sure that the 64bit JRE work fine. As far as I can tell the 64bit JVM’'s consume nearly two times the memory of a 32bit JVM so I prefer the 32 bit version but I have no idea how Windows does handle this.
One may want to add some parameters like “-Xrs” and of course “-Xmx200m” if you need 200 MB for Openfire. Using “start” one should be able to run Openfire in the background.