Mirror HD to standard TV?

We had our install last week and love DirectTV so far!

I was excited to see our HD DVR was an HR24 and the HD box a H24 - I know they're good, but don't really know why

We had a standard receiver added to our exercise room and paid the installer the $50 to mirror to our kitchen TV - both standard TVs. I'm now wondering if I should upgrade the standard box to a HD box so I can watch recorded shows in that room like we can do on our other HD box. Only downside is that I wouldn't have the mirror to our kitchen TV - or is it possible to mirror HD receiver to standard TV? (the new HD receiver would be on a standard TV also until we buy a new HD TV after Thanksgiving - I'm ok with not seeing it in HD there, just want to be able to watch my recorded shows while I'm on the treadmill).

Hopefully we can just order the HD box and do the self install if we can mirror with the mirror hardware installed and it wouldn't cost us anything extra in price, as we would return the standard receiver.

Thanks.