To investigate whether a creative writing unit in upper secondary education would improve students' creative as well as argumentative text quality and to examine whether it would change students' writing behavior, we tested a creative writing unit based on encouraging writing in flow by using divergent thinking tasks. Four classes (Grade 10) participated in a switching replications design. Students received either creative writing instruction (CWI) or argumentative writing instruction (AWI). Key stroke logging software recorded students' writing processes, their Creative Self-Concept (CSC) was measured, and text quality was rated holistically. Students were positive about the design of the creative writing unit and the lessons. The effects varied per panel. The first panel showed that CWI had an effect on creative text quality compared to AWI, while AWI had no effect on argumentative text quality, compared to CWI. This pattern indicates a transfer effect of creative writing instruction on argumentative text quality. The transfer effect was moderated by CSC, with larger effects for relatively high CSC-participants. The second panel did not replicate this pattern. Instead, a crossover effect was observed of CWI in panel 1 on the effect of participating in the unit on argumentative writing in panel 2, most pronounced in high CSC-participants. Students' creative writing speed decreased in the first panel, except for students with a relatively high Creative Self-Concept, and then increased in the second panel. Our findings may guide decisions on incorporating creative writing in the curriculum.