What do you mean by "supposedly go bad" ?
Without knowing the details of your TPE engine it will be hard for folks to help.
I doubt you are wearing out BR8ES plugs unless you ride many, many hours. Maybe it's not tuned correctly? Plus, at $1.50/ea if you are wearing them out just throw in a new set.
The BR9EYA: for starters is a range colder (9 vs 8). Unless your engine builder suggests that I don't think you should switch.
As far as the YA, it is a special edition with v-electrode. They are "foul resistant" plugs made specially for snowmobiles.
You can read about each one here on the "specs" tab:
http://www.ngk.com/product.aspx?zpid=9582
http://www.ngk.com/product.aspx?zpid=9558
Also make sure you are using the NGK 3961 solid top version of the BR8ES (and not NGK 5422). Maybe the removable nut is coming loose on your current plugs which will cause intermittent and weak spark.
Again, it comes back to what you mean by BR8ES plugs going bad, and whether or not that is tune related.