You cannot change the state of the input without changing the contact from NC to NO. A NC circuit to GND turns the Input ON. However, if you have linked the Input to another Insteon device, like a Keypadlinc, and want an Input ON condition to be reflected as an OFF button, you reverse the Insteon commands issued for each transition. Using the SHN Utility, EXIOxx tab, Inputs Settings and Monitor tab, Insteon All_Link Group Commands Settings section, Transition Off to On is, by default, an Insteon ON (0x11) command and Transition On to Off is, by default, an Insteon OFF (0x13) command. For the particular Input, change the 0x11 to a 0x13 and the 0x13 to a 0x11. With that change, when the Input goes to an ON state, an Insteon Group OFF command is issued. When the Input goes to an OFF state, an Insteon Group ON command is issued. This affects an Input only if linked to another Insteon device. The data in a Broadcast Status Change message is unchanged and the actual state of the Input is unchanged.
The GND state being Input ON applies to the opto-isolated inputs and the analog inputs working in Digital (default) mode. However, when the analog inputs are operating in Analog mode, a zero (GND) voltage is considered OFF and a positive voltage is considered ON. I have tested and confirmed this on an EZIO2X4. I have not tested the analog inputs in analog mode on the other EZIOxx devices.