First of all, make a backup of your existing project file, and store it away in a safe place, you don't want to be over-writing it.
In your RSLogix500 project file, double click "Controller Properties" in the project explorer window.
Use the drop-down on the "Processor Type" selection to change the controller type to the new type, then click "Apply".
You will be prompted to "re-size data tables", but there shouldn't be any problems.
The 1400 has larger I and O image files than any model of 1000, so the program should convert successfully.
Verify the project, and check at the bottom of the screen for "Verify has completed, no errors found". If you have errors, you will have to resolve them manually. It may be that some instructions used in the 1000 model are not available in the 1400.