Substance Sampler 4.2 has a new feature for linear upscaling. Although I would like to see the option of only upscaling a masked region that and the option of removing blur from areas of an image which typically is done using upscaling.
Just to be clear, the new feature coming with 4.2.0 isn't a linear upscaling, but an AI upscaling, which is usually a lot more useful.
That being said, it isn't possible to upscale only specific parts of a texture. Basically, the upscale allows to grow a bitmap to a bigger resolution (e.g. 1024 to 2048). Therefore, you can't take a specific part of the map and increase its pixels, because it would break the entire resolution size.