Having gone through a generally similar situation recently, here's what you need to know.
If you choose to *replace* (as opposed to *upgrade*) the problematic HR24 receiver, then you'll receive whatever latest receiver for that model there would be and you would continue operating contract-free. As far as I know, the only fee DirecTV would charge you is $19.95 for shipping kit to send back the problematic receiver (assuming it's one they want back as opposed to recycled).
If you choose to *upgrade* any receiver, you'd want to go after an HR44 Genie receiver (HR54 isn't available as far as I know unless you're going 4K) and keep the good HR24 as backup in case the Genie goes down. In this situation, your account would go back on contract. Also, if you don't already have a SWiM-based system (which can be Googled, if you don't know what that is), then you'll have to have pro installation to get that going (essentially means that you have exactly one physical coax cable to each receiver, as opposed to a separate physical cable per each tuner of each receiver). Charges would be dependent upon your particular account standing with DirecTV. I can say, based on personal experience with the Genie DVR system, it's simply fantastic, and after first getting it a few years ago, we've never had an issue with it.
Also, if you do choose to upgrade, make sure that you think about your expected topology going forward -- as in, the number of receivers that you'll want for the foreseeable future (each and every change to number of receivers modifies your contractual obligation). In all honesty, if you were itching to get a third receiver anyway, I'd go with a wireless Genie mini, because even though they charge $90 (or whatever) to professionally install a Wireless Video Bridge (they don't rely upon any customer-based wifi), it really is excellent and works well. The only drawbacks with any mini (wireless or wired) is that if the Genie goes down, so does it, which is why it's wise to have at least one non-Genie-based DVR on hand as backup.
Enjoy the decision!