I cannot stress this point enough for peeps
Running out of date forum software is like running an out of date virus software - not a great idea. There is a long long list of changes over the version releases, and any hacker worth his salt, will just have to locate older forums and start injecting malicious codes. If you have a great forum then its not wise to take the risk that it will not be comprimised, so my advice would be update the software
Okay if you are running the FREE version, you may lose some former functionality, but its a small price to pay for security. Alternatively, you could always opt to buy the software, and/or hosting packages on offer here, and get the best benefits of having a realiable web hosting company taking care of you