Is it possible to update the firmware on the original My Cloud to ver 2?
No. If one attempts to flash a v4.x My Cloud with the v2.x firmware through the Dashboard, the firmware upgrade process will stop and generate an error telling the user the firmware (v2.x) is incompatible.
Edit to add: Besides the two My Cloud versions apparently having slight different hardware, the Linux OS used to run the My Cloud is also different between the two versions. Debian version on v3.x and 4.x while v2.x uses a Linux with Busybox version.