The first thing I did was add a new property to my Kind called version, next I added an __init__ constructor for the class. I had previously thought you were not supposed to do that with App Engine, but Nick Johnson on the irc channel said as long as you are careful it would be the right thing to use in this case.
The new constructor has a version check, and if it's the old version the upgrade on the entity is performed. The super call to the base constructor must be called first.
#Since this is the first time I am using version just do a boolean check
Next I changed my code to use the new version of the entities.
Then I set up the code that creates all new instances of the Kind to use the new version number.
Finally I will (I haven't done this part yet) run a mapper over the entire kind that just has to load the entity and save it.
That last step is technically not necessary, but by doing that I will guarantee that all entities of the kind have been upgraded to the new version, and once that has been done I can remove the version checking code from the constructor.
You do not have to use a version number to decide that an upgrade is necessary. You could also use an implicit method of upgrading, such as checking if a new property exists on the entity in the datastore and if not then run the upgrade. For my use case I decided that the explicit check would be simpler and more robust.