Newb question about migration

So, I know this is probably elementary, but it's a serious question, so don't laugh.  Say I'm migrating a local account to a azure account that already exists, and has existed for a long, long time.  Does the migration from local to azure delete anything in the azure account?  The documentation only says its safe for the account being migrated, but makes no mention of the account it's being moved to.  I'm thinking that they assume that you're using a brand-spanking new account, which is why I need to ask this question.

...Partially?  I still have some concerns that I'd like you to try to address, if you have the time.  To clarify, I'm working with azure active directory, yes, but also OneDrive.  So, merging an existing local account that has an existing OneDrive with and existing azure active directory account that has an existing OneDrive doesn't merge the two OneDrives?  Possibly wiping one or more crucial documents that may or may not have the same name?  Is that what I'm hearing?  Because that's my main concern, here.