Can I switch from single to joint honours after applying?

So I am applying for Economics at Bristol, Manchester, Sussex, Warwick and Edinburgh, and they all offer slightly different joint honours including Economics that I would be interested in. However, I don’t really have enough time to thoroughly research each joint degree before applying. I was wondering if I could apply for simply BSc Economics at all 5 universities and once I have decided, switch to a joint honours degree after receiving my offers? I went to an open day at Southampton and they said that they can guarantee you a place in a joint honours degree if you decide to switch but I’m not sure if this applies to all unis? Basically, I’m asking if it’s fine to apply for BSc Econ at all my options in order to change to joint honours afterwards?