How can I cycle in winter while not being able to change work clothes?
I've had success wearing legwarmers under trousers during winter.  

The main advantage is at the far end you can either wiggle them down while standing, or drop your pants in the toilet cubicle and shove them down around your ankles for comfort.  

This works well with overshoes, in combination with a decent jacket and gloves.  I also wear a neck buff and/or helmet liner.

Try sitting nearer the window or door if you need a cooler breeze.