Well, it is no longer news that https is now available for custom domains on blogger and migrating is very easy and fast. Migration will only take less than a minute and you don’t need to worry about redirects and as everything will be in place.
How To Migrate To Https On Blogger With Custom Domain
Log in using draft.blogger.com.
If you have more than one blog on your dashboard, select the blog you want.
Go to Settings
Change HTTPS availability to Yes.
If you can’t change HTTPS redirect to Yes, close the page and go through blogger.com. After that, you can change it to Yes.
View your blog on your browser and you will see that your https:// will appear on the URL. You can also check old post URL and see if you redirection is working fine.