I'm studying epistemology, and I want to use reason and language as tools for carrying an investigation. How do I discuss the subjectivity inherent in change and progress, and also whether change and progress has the ability to be objective within the areas of math and ethics or natural sciences? Is there philosophical texts which cover this ground, and if so, what are they?
Given your penchant for using formal tools (reasons and language) to study Epistemology, I think one good source of information is the LMU site for Mathematical Philosophy. They have a bunch of materials including pdf documents, video archives as well as introduction to several research areas that hinges on formal methods of math and logic.
As for your second question, the text that resonates with it (to me) is Kuhn's The Structure of Scientific Revolutions. Although it appears as a history of science project, Kuhn writes about how change progresses in science through key concepts like paradigms. Meanwhile, I think texts that explores geneology would be useful for exploring changes and progress. Something along the lines of Nietzsche's The Geneaology of Morality would be apt. There is a contemporary Philosopher by the name of Amia Srinivasan who is embarking on such a project.
Hope this was useful!