The only way to replace the old two port cells is to cut them out and plumb in the new three port cell. This requires some PVC work, but is usually easy enough for someone with only a little DIY experience.
For the cord, the first thing is to make sure the contacts on both ends look good. The connectors should fit securely, there shouldn't be any corrosion, and you don't want to see any blackened or melted plastic, which would indicate a bad connection. You can also test the cord with a multi-meter to make sure it still conducts electricity.