If you spent any amount of time adding custom components to your template, when you set the blog up originally, it's possible that the migration process won't easily convert your custom template. You might want to do a little planning, and experimentation, before you migrate.
- Most standard template objects can be created in the Page Layout editor. Try setting up a Google account, with a new Beta blog, using a template similar to yours. Add your custom features, and see how well it works for you.
- When you get the invitation to migrate your account, first make a test copy of your blog, using a copy of the template in your Classic blog. Then migrate your account, and convert the test blog first. If you like, you can easily copy the template from the Beta blog that you setup earlier.
A little extra effort might go a long way towards making the template conversion go easier for you, and keep your blog operating for your readers. It's your choice - and I know what I'm going to do.
(Edit 12/14): This week, having gotten the tap on the shoulder, I did a dry run migration, and identified some interesting issues.