The Party Migration Tool is available on the CD ROM from which BizTalk was installed at the location below: CDRoot\BTServer\PartyMigrationTool\PartyMigrationTool.exe
You will also find .chm file which will guide you through the steps that you need to take to migrate your Parties.
To give you a basic idea of the tool, as soon as you execute it:
1) It asks for the source database from where it will migrate the parties (BizTalk management db in your case)
2) Then it asks for a temporary db to store changes.
3) Check your migrated data and save changes on your binding.
Please see screenshots and more info around point 3 at the link below:
https://msdn.microsoft.com/en-us/library/hh183961(v=bts.70).aspx[
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