Adiabatic energy losses form a well-understood part of cosmic ray modulation in the heliosphere. However, shock acceleration at the solar wind termination shock and the modulation in the heliosheath beyond this shock alter the net cosmic ray energy change from that in standard no--shock modulation models. This paper compares cosmic ray energy changes in several versions of the modulation process, from the simplest Force-Field model to one with a shock and heliosheath included. Implications for observations and their interpretation are noted. In particular, the shock acceleration mechanism seems to provide a sensitive probe of interstellar spectra at low energies (≤100 MeV/n) that was not previously appreciated. The consequence of this effect on the low-energy ratio of secondary to primary cosmic rays will be investigated in a subsequent paper.