Hi Arun,
Hope you have got a chance to review chapter 2 "Upgrading " under implementation guide.
This has all the steps on how to plan your upgrade.
During the process your 12.7 environment will be unavailable.
As a best practice try to perform the task by building a test box using swing box, and try to upgrade it first. You should be able to get to know any possible issues which might likely occur on your production.
~Vinod.