I tried changing the settings of all the plans to use "No: hide this plan from upgrades and from access proposals, unless specifically included in URL" but this did not solve the issue. I must apologise as the demo account you used had not booked the Minimalist site, so that is why the fix appeared to work.
The solution I decided to use was to set all the plans to: "Yes: show automatically plan for normal upgrades (recommended)". As I am using URLs to access the plans in all cases, it is not required to manage the display of the plans through settings. I still have the plans set to: "Only one subscription at a time" so the final effect is that the member is always only subscribed to 1 plan, but they can select any plan to subscribe to.
Thanks again for all your help.