When an economy is operating at its full-employment output, it means that it is producing at its potential GDP, which is the maximum level of output that can be sustained without generating inflationary pressures.
If there is an increase in aggregate demand with constant aggregate supply, it will lead to a situation where actual GDP exceeds potential GDP temporarily. This is because the higher demand will lead to increased production and consumption, pushing actual output above its long-term sustainable level.
However, this situation is typically not sustainable in the long term. Increased demand can lead to upward pressure on prices (inflation) as resources become fully utilized. Eventually, the economy may return to its potential GDP as inflationary pressures build and adjustments are made in the economy.
To summarize: Actual GDP will be greater than potential GDP in the short term due to an increase in aggregate demand with constant aggregate supply.