Very un-Disney-like to change the rules at the last minute like this. I'm sure some people have already shipped their costumes and/or don't have alternative attire for the race. There are going to be some p.o.'ed people if they are denied at the start and not allowed to race.
http://www.rundisney.com/blog/articles/new-rundisney-costume-and-safety-guidelines-01-04-16/
"Costumes
Disney reserves the right to deny admission to or remove any person wearing attire that is considered inappropriate or attire that could detract from the experience of other Guests. All participants and their Guests may dress as their favorite character, but must follow these guidelines as determined solely by Disney.
Costumes must be family-friendly and may not be obstructive, offensive, objectionable or violent.
Guests who dress like Characters may not pose for pictures or sign autographs for other Guests.
Costumes may not contain any weapons that resemble or could easily be mistaken for an actual weapon.
Costumes may not contain sharp objects, pointed objects or materials that may accidentally strike another Guest.
For runDisney participants ages 13 and under:
Costumes and some masks may be worn, as long the mask does not cover the entire face and eyes are visible.
For runDisney participants ages 14 and older:
Layered costumes, that could conceal prohibited items, are not permitted (e.g. Jedi robes).
Costume props, including those that surround the entire body (e.g. Death Star, UP House) are not permitted.
Costumes may not reach or drag on the ground. (e.g. full-length Princess dresses)
Capes may be worn if the length does not go below the waist.
Themed T-shirts, blouses, sweatshirts, and hats are acceptable.
Acceptable accessories include: transparent wings, plastic light sabers, toy swords, and tutus. Headwear may be worn as long as it does not cover the face.
Guests may not wear masks of any kind."