I assume you had a Volume License agreement, since that is the only way for receiving Windows Enterprise serials.
There is no way to "convert" Pro to Enterprise, except by re-installation. There might be some illegal hacks floating around, but I wouldn't recommend them.
If you have a legal serial number for Windows Enterprise and you wish to use it for x64, you really need an x64 installation ISO. You can find one by searching on the Internet, which I prefer not to detail here.
However, I must remark that the serial number you have will not stay valid forever. My personal experience with an elapsed MSDN subscription was that after some time, a Windows installation using a serial number from that subscription announced itself as non-genuine.
My best recommendation is to renew the subscription you had. Using an elapsed subscription will only work on the short-term. Otherwise, you will need to recreate an Enterprise Evaluation VM every few months.