If you're certain you want to go with Windows Phone might as well take the plunge now. If you wait for the Next Big Thing you're going to be waiting forever because something is always around the corner. However, if it's that important to you, then be prepared to wait until November or December. Even with an announcement in the next few months it wont be until the end of the year before it hits the market.
There are reports Nokia that is working on an aluminum 920 successor. Additionally, indications are the next big update from Microsoft wont come until the end of the year until the absolute earliest, 2014 being more likely. I'm assuming that the next phone will still be running WP8 and will be upgradable. So I'd expect the same from current phones. With that, you're basically waiting on hardware updates. Internals are almost definitely going to feature an incremental update. A hardware jump to Samsung Galaxy S4 levels is highly unlikely. As for the shell, I don't think the switch to aluminum is all that big a deal. Might be nice, but not essential. All in all, I don't think it's worth the 9+ month wait.
Three months from an actual release is one thing, three months from a possible announcement is meaningless.