Magento site revamp issues

*Hi,
I am Suresh as a web developer. In our new project, we have to revamp and existing magneto site (1.6.2.0) to the latest.The other requirements we have is to change the skin and add few more funcationalities into the upgraded version and update the database version as well.
There is one questions for which I am very curious is :
Q1: As the project is going to take couple of months for development, how do we going to merge the changes which users will be making on the live site into the new platform ?
Example : People will be buying new products, there will be few products those need to be shipped, new customers will be added, new products, orders will be created into the system.How do we going to push the new setup on production while also retaining the latest data from production in the new platform.
Q2: We cannot just import the latest DB in the new platform as the new platform will have its own changes at DB level as well. Thus importing current production DB in the new platform will override the newly added extensions and new tables.
Warm Regards,
Suresh Sharma