How to convert mass amounts of HD video to lower definition?

instead of waiting 60 days, why not record simultaneously in standard def. most camera systems have an AV spigot for legacy monitors, run that to a standard def recorder for the archival copy. there is no regulation prohibiting multiple recording and you would have a redundancy back up if the HD failed or was damaged.

I would not bother transcoding to downsample. The unprodictive time it would take to do that is not worth the effort or frustration.

"A lot" still does not tell us how much, but the point of 30 cameras over 60 days 24x7 is a lot - even if the cameras are programmed to only start recording when motion is detected if there is lots of traffic in/out the store.

If the video needs to be kept "forever" I would suggest getting used to buying 4 terabyte drives and copying the captured video from the DVR over to them... Another option is to have the DVR/NVR provide an alert when the drive is full (most have a warning system capability). When it is full, take it out of the DVR for storage and insert a new one. Last option is to use the DVR's USB connection and have the DVR write directly to an external 4 tb drive in a case.

With a transcoder.

We don't know what you are converting from or to.

I like HandBrake from www.handbrake.fr - there are many others.

How fast the transcoding happens will depend on the computer's CPU doing the work and running the transcoder application.

Using multiple computers will allow more transcoding to happen. We don't know how big your "mass" is.