ABSOLUTELY NOT!
Reasoning:
- The chances of finding an HR21 that DIRECTV is willing to activate are very nearly zero (if any)
- You will be installing a device that was likely manufactured more than 17 years ago, using the technology of the day (the HR21 is perhaps the slowest HD DVR DIRECTV ever released)
If the HR24 isn't working for any reason, you need to get it replaced. There's no reason that an HR24 can't be made to work in a system where an HR21 will.
What you need depends mostly on what other receivers you have (if any):
- If the HR24 is to be your only DIRECTV receiver, you should replace your LNB assembly with a SWM LNB and a power inserter and use only one cable.
- If you have other H-class receivers or a later R>=12 receivers, you'll need a suitable green-label splitter along with the above.
- If you have an R<12, HR10-250, or something older, you'll want to stay with the WB68 and figure out what's wrong with the HR24 such that it can't receive an activation signal.
I haven't figured it out from your posts, so I'm going to ask directly: what DIRECTV set-top boxes do you hope to end up with after this exercise is complete?