Overall, introduction of open textbooks can create more opportunity for faculty who wish to publish commercial material, because faculty who wish to write commercial (for sale) material will be able to leverage open content by providing commercial (for sale) addendums to that open content.
If the textbooks that are available for sale have a reasonable price, there will be little impact. However, if you can buy a printed version of an open textbook for $20 when the equivalent commercial version sells for $120, it will cause people to change.
Availability of open content and textbooks can potentially increase class enrollments by providing students with free (online) or low cost (print-on-demand) open content. Thus, students who have traditionally not been able to enroll in a course because the textbook cost is too high, will no longer encounter that barrier.