As Voice User Interfaces (VUIs) become widely popular, designers must handle new usability challenges. However, compared to other established domains such as Graphical User Interfaces (GUIs), VUI designers have fewer resources (training support, usability heuristics, design patterns) to guide them. On the other hand, GUI-trained designers may also be solicited upon to design VUIs given the increased demand for such interfaces. This raises the question: how can we best support such designers as they transition from GUI to VUI design? To answer this, we focus on usability heuristics as a key resource, and conduct several workshops with GUI design experts, exploring how they map their design experience onto VUI design. Based on this, we suggest that the "path of least resistance" to transitioning designers from GUI to VUI may be the adaptation of familiar resources and concepts (such as GUI heuristics) to the VUI design space, instead of the imposition of novel VUI-specific heuristics on GUI-trained designers. This finding can inform the development of design resources that can support the increase demand for VUIs.