It's a giant pain, but this is how I'm migrating multiple question banks. If anyone has found a better way to do it, please let me know because it's driving me crazy.

1. Create a Classic quiz and name it whatever you want the item bank to be named in New Quizzes. One thing to note here - all of the item banks for all the classes I teach are in the same massive item bank area in New Quizzes, so I've started including the course code at the front of the item bank name. I don't know if that's universal or just how my institution has it set up, but it's a little annoying.
2. Move all of the questions for that bank into that quiz. I click on "Find Questions", navigate to the bank I'm migrating, and then click "Select All" and add those to the quiz. Don't do the question group, the migration will fail. I've also tried making one big quiz for export and then sorting out questions into their proper item banks later, but the two big issues I've run into with this method are:
   1. The order of questions might not be maintained, so questions from each topic/chapter may not stay grouped together, making it harder to find the questions for the right bank
   2. There's no way I've found to move multiple questions at the same time in the New Quizzes item banks, so you'd have to repeat all the clicks to move each question individually. Either way, it's endless repetition and clicking on things; for me, I prefer to keep the questions for each bank grouped throughout the transfer.
3. Save the quiz without publishing, since it's really just there for transfer and not for students to take.
4. Go to Settings, then click on Export Course Content.
5. Select Export Type - Quiz.
6. Uncheck everything, then select one quiz to export (I've already tried doing multiple quizzes in the same export, it either fails completely or only imports the first quiz's questions to the new item bank).
7. Export the quiz and download the file from Canvas.
8. Go to any New Quiz in your assignments list or make a new one for importing purposes.
9. Click on the three dots in the upper right corner, then click Manage Item Banks.
10. Click on the +Bank button to add a new item bank (I know, I know, I'm importing a bank so I should be able to just import it, right? Nope. I need to have a bank made in order to import the questions into it).
11. Name the bank whatever you want, it'll get changed to your quiz name when you import the questions.
12. Find the item bank you just made (they're listed in alphabetical order) and click on it.
13. Click on the three dots in the upper right corner and then click on Import Content.
14. Find the export file you downloaded and drag it to the popup box to import the questions.
15. Repeat this entire process for every. single. question bank.

There has to be a better way than this. It's maddening. 

Oh, and if you ever used multiple dropdowns or multiple fill in the blanks in Classic Quizzes, make sure to check on those questions in the New Quizzes item bank. The formatting for those questions gets seriously messed up.