How likely am I to be able to switch courses at the start of first year UCL

My firm is imperial EFDS 2 A stars, insurance UCL economics and statistics 1 A star 2As. If I only met the requirements for UCL I would much rather do a more maths heavy course (e.g. maths with econ, maths with statistical science, maths with management), but these courses all have a 2 A star requirement. how likely is it for there to be a vacancy and for me to be able to switch courses at the start of y1?