I doubt you could do this at the DNS level. Reddit probably loads static assets from `reddit.com`.
You could perhaps intercept the HTTP requests, but that would require decrypting SSL, so you'd have to MITM yourself if you wanted to do it at the network level.
You'd replace the DNS request for `reddit.com` to some device that intercepts the traffic. That device would redirect the HTTP request to `old.reddit.com` if applicable; static assets would be routed to `reddit.com`. I don't know how things like HSTS and certificate pinning fit into an idea like this.
Doing this at the device level is probably easiest. I've used Redirector [0] in the past and it works well.
[0]: https://github.com/einaregilsson/Redirector
