No, the ECDH curve is not derived from an RSA public key or a key pair. This is a different kind of certificate. TLS supports DH and ECDH where the (EC)DH key pair is fixed and the public key is part of the cert, just like ECDSA or RSA. An example of such a ciphersuite (from `openssl ciphers -v`):
```
ECDH-RSA-AES128-SHA     SSLv3 Kx=ECDH/RSA Au=ECDH Enc=AES(128)  Mac=SHA1
```

Very different from:

```
ECDHE-RSA-AES128-SHA    SSLv3 Kx=ECDH     Au=RSA  Enc=AES(128)  Mac=SHA1
```
Almost identical, except the second is ephemeral; particularly note how the authentication algorithm in the former is ECDH. This is because you have a signed certificate with an ECDH key (the `xP`) in it. For this kind of cert, changing the curve is a nonsensical operation; the private key is a fixed scalar, and the public key is a fixed point on a fixed curve. As I mentioned before, much like DSS certificates, almost nobody has these (even fewer certs than ECDSA certs, which at least Google is happily signing for itself).