Now come on don`t try the double joke trick, BaartRice is not going to fall for that one, he is not as stupid as you think.
You must change ALL the winter air and change it for spring air.
Winter air has a greater density due to moisture and will form a heavy air layer on the outer surfaces of the tyre while at speed. This causes the tyre to run as though it is slightly out of shape and can cause side slapping, which as you know, it is important to avoid.
I change my winter air at the beginning of every riding season and have never had a problem with side slapping.
Steve