In my ever so humble opinion, national hunt horses ratings aren't as important as they are on the flat. The margins of victory can often be a lot bigger and it is much harder to get a grip on. The trips are further, they don't factor in 'mistakes' or not jumping straight, just loads of additional variables that need to be factored in. I mentioned earlier (or on another thread, that I do like the best hurdler when it goes chasing) and if it lends itself to my thinking then I like it, but I wouldn't ever use the ratings as a solid rule. If Old Guard starts jumping and looking 'a natural' (which he may not as Faugheen says about being flat bred) then his hurdle rating will be based upon the new code anyway.
I don't have the information now, but I imagine Sprinter Sacre's rating changed dramatically after he started popping a few fences. The fact that 'a fence' can bring out such improvement gets proven time and time again, so despite me liking to use hurdle form, I think chasing is so unique that seeing is believing.