Whole Home Equipment Upgrade - Do or Don't

First off it is a scam, DTV never calls to upgrade your equipment.

Second reason it is a scam, upgrades always come with a 2 year commitment.

You can try running a search for          clearmybox     that will dump the guide data and other things, it will take a day or 2 to rebuild, any recordings set to record will, on each receiver.

I see no reason to upgrade, but if you do want to, all DTV has to run that may TVs is the HS17 Genie2 that is a client only system so you will lose all the HRs and get 7 mini clients all can be 4K if all TVs are 4K at the time of install but only 2 at a time can stream 4K.

Only the 4K clients C61K or the C71WK can output 4K, yes all minis clients can watch recorded programs or live channels provided there are tuners available for each, recording channels uses a tuner so the HS17 has 7 tuners so if you are recording 3 channels at the same time you can only watch live TV on 4 clients.

AFAIK DTV no longer has any HR24s in stock.  Some online DTV retailers have them at the full upfront lease fee but not sure if DTV will activate them.

Per standard policy, no, although there is some evidence Directv has been replacing faulty HR24s recently for some people. With the HR54, you are limited to two 4k streams anyway so another C71KW would be a good replacement for the Hr21 if you call Directv to replace that one, tell them it's faulty (won't power up maybe).

If you tell Directv one of your HR22s is faulty you MIGHT get an HR24 to replace it but there is no guarantee.

The other obvious path is replace everything, with the HS17 replacing the HR54 and all of the HR21/22/24 boxes with clients (including at least one more C71KW). That's what Directv would probably try to get you to do, because that's the current :"standard " solution. But as Juniper says, that would be a real downgrade so none of us would recommend it. 

If it were me I would get Directv to replace an HR21 with a C71KW and leave it at that

The HR54 can only stream to one 4K mini at a time, only the HS17 streams to 2.

OK so lets have another look at this. 

There's nothing that would cause multiple HR series boxes to all become slow. They don't share anything that could affect them all, except they all use the same software. If you go to the server/client model, then of course the software will change but there's no guarantee it will be faster. One thing juniper mentions which can be a deal-killer with the client server model is that because all the clients share the same playlist you can't separate the recordings, everyone has access to all the recordings so each user can delete anything. For me this would be a deal-killer, 

In any case I  doubt that Directv would be willing to replace all your equipment free of charge.

What would I do? Stick with your current configuration. If one of the HR  boxes is notably slower than the others, replace it with a C61K That will give you two locations where you can access 4K. Only one 4K stream can be active at any time, although the "other" C61K will behave just like a non-4K client.