They have relaxed the dress rules to allow any footwear in any enclosure.
If you are in the Silver Ring I would seriously suggest you consider wearing wellies. I was in the Silver Ring this morning, before racing, and the ground was getting very muddy off the walkways – after this afternoons rain and crowds I expect it will be Glastonbury like by the morning.
After my walk round this morning my shoes were covered in mud and I had mud splashes well up both legs.