Yes if you call and tell them you purchased a 3DTV they will upgrade you from a HR20. You might not get a HR24, but you will get a 3D compatable HD DVR.
If it is free, then I would go this route.
There is a good chance it will be an HR24 just not guaranteed.
However, if I had a choice of paying $200 for an HR24 or just getting a free HR21-23 I would take the free option.
All new and refurbished receivers sent will be leased no matter if it is free or you pay for the upgrade.