Re applying to the same university but for a different course

I decided I did not want to go to any of my original choices and so declined them all and applied through UCAS Extra to another University closer to home but for a different course. I got an offer back, however, I'm not completely sure I can get the grades and I don't want to risk not getting them when I don't get an insurance choice with Extra. I wanted to decline the offer and reapply to the same university but for a different course but my main concern is that they might not take too kindly to that? Maybe I'm overthinking things but is it likely that they will see me declining their previous offer negatively and therefore not want to give me an offer for another course?

You might not get an insurance choice through Extra but you would still be eligible for Clearing if you don't get the grades and that would enable you to stick with the course you want? But if you're sure you want to change, I would just give the uni Admissions a ring and ask if you can switch courses, I'm sure they won't mind doing so if the other course still has vacancies. But have a good think about it first