To reshape an associative hatch object in AutoCAD, you modify its boundary objects. To reshape a nonassociative or unbounded hatch, you modify the hatch object itself.  But there are some options on the grips, when the hatch object is selected, that might make your life easier if you don’t want to rehatch an area. 

When the hatch object is selected, if you hover over one of the grips, you will see three options:  Stretch Vertex, Add Vertex, and Remove Vertex. (If you click a mid-point grip of a line, you will get the option to Convert to Arc and vice versa.) These options give you more flexibility to adjust hatch objects. 

I discovered this trick by accidentally hovering over a grip one day.  Hope this helps.  I use it all the time!

Notes from Cadalyst tip reviewer R.K. McSwain: Tip 3521, Recreate Hatch Boundaries, describes how to generate a hatch boundary if it's missing, but as Nereida points out here, the hatch object itself has grip points with polyline-like editing capabilities. So with or without a hatch boundary, you can quickly edit the hatch object without having to recreate the hatch from scratch.