What model is the 'old' box? AFAIK The recordings are encrypted so they only playback on the Freesat box. You can connect an external hard drive and copy SD recordings to it, as part of the process they convert to .ts files that then can be played on other devices. HD recordings excepted (can't convert them to .ts)
It might be possible to put the old hard drive in a caddy and connect it via USB to the new Freesat box and then they should play ok. I don't know of any reason that wouldn't work.