After hats and sunglasses were added as appropriate parts of the dress and grooming standards for LDS missionaries, another major change has been announced. To prevent disease in certain missions, especially diseases carried by mosquitoes and other insects, sister missionaries are now allowed to wear dress slacks in certain areas.
With the increasing prevalence of serious mosquito-borne diseases in certain areas of the world, you are encouraged during seasons when the risk is high for mosquito bites to wear dress slacks during regular proselyting activities for additional protection. In areas within missions where it is not culturally acceptable for women (including sister missionaries) to wear dress slacks, you may wear ankle-length skirts. In such missions, you are also strongly encouraged to apply insect repellant containing DEET to exposed skin and to apply insecticides to the outside surfaces of outer clothing to help prevent insect-borne diseases.
Dress slacks are to be modest and look professional. Full guidelines can be found here.
Here is the list of approved missions now allowing the change.