From what I can gather from this TechNet article, you might be able to disable and re-enable Bitlocker. Forcing Bitlocker into disabled mode supposedly leaves the drive encrypted, but with the drive master key on the drive protected only by an unencrypted symmetric key. Enabling Bitlocker again protects the drive master key in the normal way.
Looks like you might be able to transfer the key material to the new TPM from the command line if that doesn't work.
Worst case, you have to turn off BitLocker, decrypt the drive, turn it back on and re-encrypt the drive.