If you have Plesk version for Debian 3.1 and you upgrade Debian to 5 version your Plesk will be broken because version for Debian 5 should be installed.
If you have Plesk version for Debian 3.1 and you upgrade Debian to 5 version your Plesk will be broken because version for Debian 5 should be installed.
Install Debian 5 server with Plesk and migrate all content from old server to new via migration manager or with backup/restore procedure as it is described in Migration Guide.