Around 50/50, I'd say.

UA has 38 789s: 19 Polaris, 16 old config, and 3 in mod. By later this year, that split should be closer to 22/13/3 or 24/11/3--the later in the year the better your odds. However, you also have to consider that some of the Polaris 789s are dedicated to certain routes, such as EWR-JNB and even SFO-NRT--basically any 789 route selling P+ as a separate cabin is more or less guaranteed to be on a Polaris 789. Not sure how many of these routes are, but if you start with 22/13/3 and take, say, 8-10 Polaris aircraft out of the equation to operate those guaranteed routes, you end up with something like 13/13/3, or 50/50 odds of Polaris.

Of course, the usual caveats apply: further equipment changes may take place well in advance of your flight, and other operational factors close to the day of departure may impact your odds one way or the other. But hopefully that back-of-the-envelope analysis is at least somewhat helpful.