Yes, but we do not recommend it!
Technically it's possible to go directly to the BOY_E0_CONFIG table and correct the demo field for the corresponding gateway to 'Y'.
Authorize.Net - U_AuthDemoMode
Cybersource - U_CYBERSOURCEDEMO
Eway - U_EWAYDEMOMODE
Moneris - U_MONDEMOMODE
Cayan - U_CAYANDEMOMODE
ProPay - U_PROPAYDEMO
Square - U_SQDEMO
This will force the gateway back to demo mode.
Doing so, however, you should be careful that you don't have any live data in the database - any tokens etc from the live environment that you would like to keep.
Once you switch back to Live mode, all iPayment tables will be cleared.
We do this to avoid Live and Demo mode data to be mixed, as this creates a number of problems and becomes very confusing for the user.
We obviously do not recommend switching back and forth between Live and Demo and mixing data from the different environments, which is why it is disabled - but if you are aware of the risk and want to do it, then this is the way to achieve it.