“What about my mail merge tags? Will those change?"
SHORT ANSWER: No :)
MAIL MERGES IN G SUITE: These should not be affected by the migration. In fact, in addition to the Microsoft Office merges you may already use, G Suite also has plenty of cool features available in various G Suite products and 3rd party add-ons.
(NOT SO) SIMPLE MAIL MERGE TUTORIAL (for the pros): Check out this video to learn (https://www.youtube.com/watch?v=BUepbBbK6lg) or see the full tutorial here (https://developers.google.com/apps-script/articles/mail_merge#section-1-preparing-a-spreadsheet-and-running-the-example).