Sorry for the late reply, just noticed we had some pending posts we needed to manually approve!
I think this happened in an update since you originally posted this, but you can now save the sprite you are working on with a different name and it won't overwrite the original save file. So Save 'Mao_Summer' then change the outfit into winter clothes and save it as 'Mao_Winter' and both sprites should be in your saved sprites list!