I'd guess the GTV is very similar setup to the 145, here is what I have found from my research:
CODE light on solidly - key not recognised or absent - eg faulty or disconnected aerial around ignition barrel, CODE ecu faulty, no connection between CODE & engine ECU, keys not memorised correctly or CODE not matching engine ECU. The engine will still turn but not fire. Diagnostics on the engine ECU will report engine starting is not permitted because CODE not received (link faulty), or wrong CODE.
If it flashes (after it has come on for a second or so and then goes out) - system intact and brand new (no key codes or engine ecu code memorised), no connection between the 2 ECU's, or key transponder absent/faulty or disconnected aerial.
These all have different frequency of flashes to identify the error, same as above, engine will still turn but not fire.
Swapping the CODE boxes won't hurt anything - the key transponder info & master code are stored on an eeprom, no damage can be done to either ECU, the CODE ecu will not forget what it has memorised, and you cannot force it to relearn without purchasing a brand new virgin code unit. Or I can flash it with my virgin dump to reset it
If it is a working set (engine ecu, code ecu, 3 memorised keys) then you will not need to re-memorise the keys & the master code.
The procedure is turn on with the master key and hold ignition on for 5 seconds, then take it out, put in normal key for about 2-3 seconds, key out, other normal key same as the other one, then close the procedure with the master key.
I forget the exact time wait periods off the top of my head.
You need a minimum of one master key & 2 normals to complete the key memorising correctly.
Disconnecting the battery won't do anymore than discharge the caps in the CODE ecu - it will still remember the codes as they are stored on an eeprom.