Cant move to 6 months plan

I obtained my Edgars account about 6 years ago. I was forced to take out an interest bearing 12 month plan and was told i could later move over to a 6 month plan (no interest). However I contact the support team and I cannot do this. They stated it has something to do with my affordability. I then suggested that they just half my credit limit and over 6 months this comes to the exact same affordability. But no this isn't possible? Why cant my account be moved to a 6 month plan anymore (with a credit limit reduction)? It should be a simple change.

Btw they can easily up my credit limit, but when it comes to lowering it, it becomes a problem. The only thing I can think of is that Edgars (Edcon) is trying to ***** me on interest here. And for anyone else that wants to move over