First year Maths student here, let me know if you have any questions.
For my course at least, anyone doing any Maths degrees are grouped together and take the same modules - there's about 350 of us in our first year doing different courses like MMath and BSc with placement, etc. This allows you to change courses at the end of your first year without any issues, though it may be subject to conditions. You need to meet certain criteria to be able to go on a placement, but this will all be explained at the start of the second semester so dot worry, just do the best you can throughout the year. I'm currently on the BSc but hoping to switch to the placement course (pending on my grade at the end of the year). I'm not 100% sure but I imagine the Economics course wouldnt be too different in terms of structure.