Mark, you may want to email every existing user the new EULA, then, give them a link to confirm that they agree with the new EULA, if they do not confirm by a set date, revoke their BUID until they do.
Quite apart from seriously annoying thousands of people who have changed their email address since they bought Blitz, or who have dodgy spam filters, or a hundred other things, it would be entirely illegal to force people to agree to something retroactively and take away what they'd paid for in order to enforce it.
I tend to agree with Skidracer. Reiterate copyright and your standard non-liability disclaimer.