Such changes would be found in economics, politics, social etc. A radical wants extreme change and can be found on both the left and right of the political spectrum. The re-introduction of capital punishment would be a radical policy most associated with the extreme right of politics. A wealth tax would be a radical policy most associated with the extreme left of politics. What is a major change for one person, might not be so for another.
Radicalism is now, for right or for wrong, most associated with the left of politics.
The word radical has been used in the past to discredit policies as opposed to supporting them. Socialism This is a political belief that believes in the group ownership of the means of production and distribution of goods. It aims to eliminate poverty and introduce co-operation and social responsibility. One basic plank of socialism is to introduce a more equitable distribution of wealth and give everybody opportunities. However, over the years, numerous schools of socialism have grown, some wanting a more radical approach with others seeking a more conservative and restrained approach - seeing what works and then building on those policies and jettisoning those that do not work for the good of all.