Will it be hard going? This depends on the age of the building and original mortar and brick type, and how well the mortar joints had been raked out when the pointing had been done, and how hard the pointing mixture was.
If the bricks and original mortar are soft, and the mortar was lime based, which usually means built between the wars and before, then that mortar will not be hard to remove. If the pointing was raked out just sufficient to lay the pointing mix on the joints, then the removal will not be too difficult.
If the pointing was done by raking out to the recommended depth - the same depth as the thickness of the mortar joint, or slightly more, and done with a strong hard mix of portland cement and sand, then it will be hard to remove, as it would if the original mortar was a hard mix. If the bricks are softer than the mortar, they will be damaged if a chisel is used.
Chasing out mortar joints using power tools is also problematical, not least because of the height above ground that the work needs to be carried out, the high speeds of cutting disks, and the amounts of dust produced, which reduce your view of the work, and a horizontal chase could wander into the brickwork above or below.
An SDS drill could be used to remove some of the mortar if it is hard, drilling holes as closely spaced as possible, before cleaning the joint out using the plugging chisel, or a bolster chisel. Oscillating multitools with carborundum semicircular blades can also give more control than rotating grinders, and give out less dust. A platform to work on, rather than a ladder will also make the job easier, and safety goggles, mask and gloves will help protect hands and face etc.
