Hey!
I'd recommend emailing them now rather than waiting. Firms will usually prefer the transparency, and it's much easier for them to move you into a different pool of candidates rather than trying to reorganise when they've given out offers (as at that point, they may need to ask people to swap to the other scheme if there are set numbers for each).
If you politely explain that you have have another WVS and would like to be considered for the spring scheme instead, they're normally understanding - I did this last cycle and asked to be considered for the spring scheme instead, and still ended up receiving an offer.
I would just send a polite and appreciative email, as I'm sure it is unlikely to be a problem