That's called "level of detail" or "LOD" for short.
You can not change that inside hammer far as I know. It's a feature compiled into the model itself that (ideally) swaps out a higher poly model with a lower, less resource intensive version at distances where the high detail wouldn't be seen. In theory, the model maker will always set it so that the transistion is difficult to see during gameplay, but sometimes they'll opt to make it more aggressive (for example, if the model is already fairly high poly and/or intended to have multiples in a map).
You can change it per client using the console command/config
r_lod however. But not inside hammer, and not on a per model basis.