I need help regarding oyster max

Im desperately after some advice please. I have an oyster max tandem. I am currently using the upper seat for my toddler and carrycot in the lower position for my 4month old baby. We are going away in the car at the weekend and have very limited boot space so I want to use thd lower seat. The issue is that my lower seat is the old sTyler and doesn't lie flat and my toddler refuses to go in the lower seat. Has anyone swapped the upper and lower seats? They physically fit but state on the chair itself to not switch so I'm apprehensive in case it is unsafe. Tia