when you save an assembly as a part you get a part model with different bodies corresponding to the relevant parts on the original assembly.
As far as I'm aware (I'm on SW2015), you can't keep the features of the single parts. You could try to run 'Feature Recognition' when opening the new part file (created from assembly) but it's highly unlikely that you get the result you expect.
If you want to keep the features for future modification purpose, you can rather use 'Direct Editing' for moving faces, holes, etc.
I use 'Direct Edit' a lot for future modifications on released parts; it has the benefit that you can always go back to the previous state. The only limitation is on the tapped holes!
Hope it helps.
As Sergio Monti explained, saving an Assembly as a Part and preserving the features will be problematic. If you could explain why you want to do this perhaps someone could suggest an alternate workflow that would produce the desired results.
It's look like a mirrored part you try to make it with this comand?
This assy is too simple to try to figure out how to. Just edit the original part to look like this.
Take a look at this - 2018 SOLIDWORKS Help - Multibody Overview
You could take your original part and mirror it to make a multibody part. Keep the mirror feature at the bottom of your feature tree, you can go back up the feature make any changes and all will be mirrored.
Hope that helps