It's really a new install except for already having an existing key. However, a lot of people thinking they have the key find they did not include the required private key in their export - therefore really needing to create a new key. I'd suggest doing the new install, and then importing the key. If the private key is really there, you can then right click on the key in All Keys of PGP Desktop, select Properties, and set Trust to Implicit. If you can't set Trust to Implicit, it is only a public key and you will have to generate a new key for him.