I personally cannot help you because I do not have that kind of knowledge, but
some things that will need to be known for someone to know if they can help
you or not is do you have administrator access/permissions to the forum?
When is the transition to the new software to take place. What kind of
timeline do you have to work with?
If you have the necessary administrator permissions you might also try the
below forum for help. There may be other forums, this one came up in a quick
search.

<http://phpbb2refugees.com/index.php>


If you do not have administrator access to the forum the best you can hope
for, I think, is to make a mirror of the forum, but it would be "read-only".
Things like search, post new topic, etc. would not function.