I am half way through doing this now... (almost 24 hours so far
)
What I did was...
1) Create a copy of my 6.5.x database
2) Create a new folder for threads 7.x under my domain
3) Install threads 7.x into new folder
4) Run impoter on copy of 6.5.x database.
The original forums stay up and you should be able to log into the new ones as soon as groups and members have been imported (ie once it starts on the topics)
There may be another method, but this way if the import fails, the original forums remain untouched.
The only major item remains to be done is to change lots of links
