A succulent plant is grown in a low light situation so that it is very tall. pale, and has few leaves compared to its siblings grown in full sun. Is this an evolutionary change or not, and why?

This is not an evolutionary change. This is called etiolation which is a process by plants grown in the absence of sunlight. This can manifested by plants through weaker stems, fewer leaves, longer internodes and pale to yellowish color of plants. This is not an optimum condition for plants as they cannot adapt in the changes in the enviroment.