the normal condition of an SCR is in the non-conduction mode. To make the SCR conduct a small trigger voltage is applied to the 3rd leg. Since the SCR is not conducting when the trigger is applied, the "anode-cathode" current is non-existant. Once the SCR is triggered, and starts to conduct, it will continue conduction even in the absense of external trigger voltage being applied. So your premise of changing trigger voltage sometime after conduction starts is a moot point.
The SCR is not an analog amplifier where the pass current is somehow controlled by a gate current. You have it confused with a transistor, which it is not.
The applications where the SCR is used to control current usually is based on the principal of conduction phase. by delaying the position in the waveform when the trigger pulse is first applied, that earlier part of the wave is "lost" and only the following part can be rectified. The greater the trigger delay, the less gets rectified. So the SCR can be used as a rather crude controller of AC circuit current, but it is the timing of the trigger pulse and not the trigger current that does the job.