In terms of getting the best information out of the session, come armed with questions about the specific areas of the plan that you're least confident about. Also bring a few vaguer overarching questions - such as about what mistakes people most commonly make when doing such migrations; or about what the consultant sees as the highest risk component of your plan as currently formulated. These will allow you to get information about items that you may not have thought to ask about but that your expert can leverage their expertise to recognize as potential issues.
If you have time (as in hours, not seconds) to review the feedback and process it thoughtfully before responding, that can help a lot in terms of overcoming knee jerk emotional or defensive reactions.
I also find two distinctions important to make.
The first is between critique and criticism. That someone has suggestions to improve your plan doesn't mean that the plan was bad in the first place. It just means it was imperfect - as is everything humans do (yes, even you) - and has room for improvement. I think many of us in the tech community have a perfectionist streak, but when accepting feedback it's very important to forgive yourself for not being perfect.
The second is between critique of your work and critique of you personally. For those of us who take a lot of pride and invest a good deal of ourselves in our work, when someone points out its defects it can feel like a personal attack - even when the critique was in no way intended to be directed at your person. It's very important to remind yourself of that as you read or listen to feedback.
The part of accepting feedback that I personally find hardest is overcoming the sense that it means my initial work wasn't appreciated. Knowing what your own personal pitfalls are for feeling crappy about feedback can help you look out for them and catch yourself when you begin to fall into them, so you can step back and consider the situation more rationally and less emotionally.
Finally, in the bigger picture think of this as a learning experience. Imagine the work you've done so far as a school assignment to practice what you e learned so far. You're turning it in to a teacher who is going to point out areas for improvement so that you can level up the next time and do it better on your own.