Rebuilding after a fire is hectic enough without confusion from not knowing what you can or can't do. Generally what you can do is going to be based on your individual coverage with your insurance company. Be sure to check what type of policy you have, and what it covers. Most poeple will have a certain dollar amount of coverage, that will largely depend on the value of their home.
Let's say for instance you have $200,000 in coverage. You get that amount to rebuild and replace your belongings. In that case you can build as you please with that money. You just have to stay inside of your budget of coverage money or pay for the rest yourself out of pocket. There is also policies that have guaranteed replacement. This means that if you have $200,000 in coverage but it is going to take more to replace, your insurance company will pay this too. With this type of insurance you will have to have the same square footage of your previous home, same materials used to build, and the same type of trim, ect.
As long as the changes you are wanting won't raise the cost of rebuilding your home, you should be able to make some changes to the floor plan. But this can vary between individual policies, as well as between companies. To be certain you should refer to your policy before making any changes. If it isn't prohibited within the policy check with your insurance agent to be positive that changing the floor plan isn't prohibited.