In my eyes, it is very clear: Offer me to update from stable versions to unstable testing versions. If your version is already unstable, the term 'from stable versions' does not apply.
@rejetto
I'd simply reverse the option. Rename it to Released versions only with default [√]enabled. If you disable it , you'll get unstable versions, too. If you already have an unstable version running and enable this option, HFS will (after a warning dialog) automatically downgrade to the latest stable. So you can finish your beta-test everytime you want.
To save bandwith, you could even keep always a backup of the latest stable version in the HFS folder, even when testing betas. That means, 'Keep old version (when) updating' will be extended to keep (additionally to HFS.old.exe) an HFS.old-stable.exe .
Whew, long post.