Thank you Dark_Star for asking such a good question. I'm not really sure since I have never had to have the few pieces I own resized before. However, since the resizing method of rings involves removing a fraction of the material, sanding, smoothing, and soldering the piece together, this would no doubt expose the silicone filing inside and my guess would be that it would have to be removed and refilled to the current size the ring has been shaped into.
For those who may be a little leary of having a silicone filled ring resized, there are ring guards and ring size reducers (ironically made of silicone) that can be used if a ring is too large. If the ring is too small however, you'd have to enquire of your local jewelry repair person to see if it can be effectively resized for you or not.