Hi!
This can vary between firms, but it's often possible! After securing a Summer VS, I contacted the early careers team at another firm I had to applied to for a Summer VS and asked whether I could switch to the Spring scheme instead. They agreed, and I ultimately received a Spring VS offer from them (so it isn't going to negatively impact your application!).
If you're considering doing the same, just make sure you reach out as early as possible as it makes the process much easier for them to accommodate. A polite email explaining your situation and asking whether a switch is possible is completely acceptable