CAD tip # 8177:

Though there is a "Recreate boundary" function in the AutoCAD ribbon (and in the HATCHEDIT dialog) but it performs a slightly different function - it re-constructs the boundary of an non-associatve hatch and inserts it as a new curve to the drawing (optionally making it associative with the hatch). So in the standard case you will end up with two boundary curves - the original one (still not associated) and the new overlaid one.

The closest functionality for reassociation can be achieved with the menu macro

_-HATCHEDIT;\_Assoc;