You will not be able to change your choice of theme on LSE100 once you arrive at LSE. This is not something that you should worry about when making your choice.
LSE100 is a single course and its themes are highly interrelated. Climate futures are affected by developments in AI and has huge implication for fairness in society; the rapid growth in AI is already having an impact on social inequalities as well as environments across the world; and our conceptions of a fair society can only be properly understood in the context of technological and environmental changes.
In the second half of the course you, along with a small group of your peers, will investigate an element of your chosen theme in greater depth. At this point, if you have an interest in an aspect of one of themes you did not select, you will have an opportunity to focus on your research on your chosen theme on the areas where the two topics intersect.
As well as the interrelated nature of the themes, all students will study the concepts of systems thinking and systems change while on LSE100. These are fields of interdisciplinary research that are being adopted by academics, policy makers, corporations, NGOs and many other groups in order to think holistically about how systems work and how this understanding can enable change at a systemic level.