Migrating Users with Migrate D2D when/if they already exist
If Migrate and Migrate D2d modules are installed with the UIs, go to Content > Migrate tab. Once you get the migration set up, there is an option under "options". Check "update" and it will just update the existing users with any information that is new or not matched, keeping the UIDs, etc. It will also update with any new users. It's a long process if you have a lot of users, but so far so good.