The source has an UPGRADING file that details any config file changes between versions. The safest way to upgrade is to use the latest hastymail2.conf.example file, edit it with your site specific changes then rebuild the hastymail2.rc file (so basically re-install the config and code, you can leave the settings/attachments dirs alone). From RC7 to RC8 a few config file entries were changed so you should take a look at the UPGRADING file. If you don't use those particular settings you can upgrade by simply swapping out the hastymail2 directory with the RC8 code and re-using your old hastymail2.rc file.

Jason