Increasingly complex abstraction and composition constructs have been proposed for Petri nets, sometimes to the point that the quantity and diversity of net inscriptions seem to obscure the proclaimed advantages of Petri nets graphical representation. These constructs are oriented towards system synthesis but typically overlook the necessity of future model modifications due to unanticipated changes in requirements. These can imply modifications across several of the initially composed modules. This paper proposes the use of two simple composition operations, net addition and net subtraction, that allow the introduction of the necessary modifications without being dependent on the net class or composition constructs in use. The operations can even be used concomitantly with the existent composition constructs. Three application examples are also presented which illustrate the use of the proposed operations in classic settings.