Understanding update in Heat orchestration templates

Hello Team we are using Heat to automate orchestration of our application nodes. So please consider a case: I am running 3 nodes in the heat stack. And let's say I have made some updates in the the application code over these 3 nodes. So when I scale up the stack orchestrates and add another node but with the original image. To deal with this if I update the stack template heat rebuilds the original 3 nodes as well with the new snapshot image which makes my application to be unavailable during the update process. It would be really helpful if anybody could put in some ideas around this on how work this case around? Thanks in advance Uday Dikshit