It is well-known that people diagnosed with autism spectrum disorder experience difficulty coping with change. However, the extent to which this applies to small changes in user interface design is not known. Through exposing autistic (n=11) and control (n=10) users to seven controlled, cumulative design changes within an e-calendar appointment creation interface, and systematically recording usability scores, a bespoke comfortability score and qualitative responses, we confirm that the autistic discomfort around change extends to small changes in user interface design, negatively impacting system usability and comfortability scores consistently in the autistic group for some tests. To widen the applicability of our findings for industry use and promote further research in this under-explored yet critical area, we also transform our results into preliminary non-exhaustive design heuristics for reducing negative impacts of interface design changes in autistic users.