Each zone has a limit to how many buildings can be upgraded ... which is marked by the yellow hat. So even if you have say 15 regular residential buildings which are not fully upgraded, the system will display at most 5 yellow hats (new zones exempted). Pressing the blue stop sign shifts the ability to upgrade to another building of the same zone which is also ready to be upgraded but not showing a yellow hat ... there is a waiting period before you can upgrade that building just like when you select to change the materials used. If there are no other buildings ready for upgrade, it acts like you selected to change plans on that building.