I know D.O can't see this but I'm not really doing it for his benefit. The take 10lb off for a chaser returning to hurdles is illogical the way he comes to that conclusion, I've read his thoughts on that before and couldn't really be bothered to comment as it should be fairly obvious to all anyway.
D.O view is that the majority of horses improve 10lb from hurdling to chases therefore when they come back to hurdling their ability reduces again. There is a case that some horses are better chasers than hurdlers and visa versa that's not the point I'd be arguing. It's this general improvement of 10lb number that's the problem as there is a more basic explanation for this. A horse most often starts off hurdling and does so more often than not at the age of 5 or 6 I know there are Juveniles but for these purposes the average of 5 or 6 will do. And lets say as an average most horses would have a novice season, and then maybe a season handicapping at most before they go chasing. Sometimes it's just a novice season. But if a horse is thought to be capable of being made up into a chaser it would be rare for horses to spend much longer over hurdles than 2 seasons if they can jump a fence. Usually over hurdles most stick between 2m and 2m4f over those initial couple of years. Then lets say the average age to start Chasing is 6 or 7. Firstly a horse start at those ages will normally go until and average age of 10 so they spend more years jumping fences than they do hurdles and tend to try longer trips as they age factors that lead to improvement in their marks. But the main point here is what age would you say a national hunt horse reaches it's peak? In most cases it's between the ages of 6 and 8. Of course there are horses who peak early and some who flourish later in life but I'd say as an average 6-8 is fair.
So In the main your average national hunt horses who goes from hurdles to fences will spend his prime physical years over the larger obstacles, so is it any wonder they carry on improving by D.O's average of 10lbs. It's more to do with physical improvement than it is to do with the change in obstacles. As I've said earlier there are horses that are better in one discipline than the other. However that's purley down to jumping and should be judged on an individual basis. I'm sure we've all seen horses come back from fences and jump hurdles slow and cumbersome or balloon over them losing time in the air. However on the same hand we've seen horses who return to hurdles and still jump them very slickly. If this is the case to deduct a horse returning from chasing to hurdles 10lb as a rule of thumb borders on the ridiculous. If a horse has reached a level of ability in his peak years over fences, returns to hurdles during this time and is still able to to jump them slickly enough that improvement should reflect in his hurdles mark.
As with all racing, every race is a unique set of circumstances, every horse in it's own way unique and in general in my view to apply any rule of thumb is costly in betting especially when the concept hasn't been thought through for all the different cause and effect.