Prompting-based user interfaces (UIs) shift the task of defining and accessing relevant functions from developers to users. However, how UIs shape this flexibility has not yet been investigated explicitly. We explored interaction with Large Language Models (LLMs) over four years, before and after the rise of general-purpose LLMs: (1) Our survey (N=121) elicited how users envision to delegate writing tasks to AI. This informed a conversational UI design. (2) A user study (N=10) revealed that people regressed to using short command-like prompts. (3) When providing these directly as shortcuts in a toolbar UI, in addition to prompting, users in our second study (N=12) dynamically switched between specified and flexible AI functions. We discuss functional flexibility as a new theoretical construct and thinking tool. Our work highlights the value of moving beyond conversational UIs, by considering how different UIs shape users' access to the functional space of generative AI models.