I never comment on this kind of subject, but this one is very important. I been part of a lot of migration and do it this way have a high risk and even Microsoft don't recommend this if you didn't test all your application. If you have only 1 application, this could be done, but in most of the company their have shared SQL with more than 1 application. The rollback will be for all your applications and not just one if only one failed. I'm not telling you is not good of the point of view technical but in the point of view business, this is too much risk.