Cisco Secure or Microsoft IAS

My company is currently undergoing an Infrastracture revamp. One of the projects I am working on is Remote Access. Currently to authenticate our remote access clients we are using an Annex Terminal Server with ERPCD. We are going to change this infrastructure to a Cisco AS5300 with Radius.

The problem we have is how can we migrate from the UNIX (NCR UNIX ver 3) based machine authenticating with ERPCD to an NT machine running Radius without loosing the usernames and passwords ?.

Also whatadvantages has Cisco Secure on Microsoft IAS and vice versa. All our new equipment will be Cisco.

Thanks