What forum module are you using? It’s possible that another module might accept an export from the older forum module you’re using in Joomla 1.5.
You’re SOL when it comes to Joomla 1.5 and security. Support for it was dropped at the end of 2012. Meaning, any new bugs or security flaws that have been discovered since then will remain unpatched. It’s a risky move to continue running an unsupported CMS version.
Joomla, and especially the 1.5 branch, has been infamous for its security issues.