the service accounts will need to be changed in the initial phase, this should be done via sql server configuration manager.

Once the instances are online you may start to script out logins and then recreate them, this will be necessary as the new windows accounts SIDs will no longer match the original windows SIDs you'll end up with orphaned database users.

Once the new windows logins have been created you may then loop through each database user and use the ALTER USER command to change the database user mapping, like so

ALTER USER [OLDDomain\bob] WITH NAME = [NEWDomain\bob], LOGIN = [NEWDomain\bob]