SharePoint Permissions Issue - Users moved from Domain A to Domain B

We have an Windows Server 2003 AD DS forest, under which we have two
domains trusted
Domain A and Domain B

We are moving users from Domain A to Domain B

We have MOSS 2007 intranet and internet SharePoint site which have
several site collections with sites ranging from 10-500 in each site
collection.
Till now the users were given permissions using DomainA\userid to all
these sites.

After the users are moved to the new domain, I have to give
permissions to the individual users once again by adding them to the
sites using the DomainB\userid account.
We have 5000+ users what is the best way to replace or change the
users permissions to use the DomainB\Userid accounts.

Are there any tools available for doing this?

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