Also worth noting that Google built an automated tool to handle the M/S to HRD datastore migration for you. It was remarkably painless.

I really struggled around 12 months ago. Lots of weird errors and it took down the service unexpectedly when trying to stage it.

Really made me regret going with app engine a few years ago.

Painless eventually; the first versions didn't work at all, at least on the project I was involved with. Fortunately by the time you had to switch, it worked very well.

Being first to migrate to the hot new infrastructure has never worked out for me in any project.

I was mildly annoyed that I had to spend an afternoon "migrating" my app from the M/S datastore, even though I never used it in the first place.