Yes. If you are doing it for tax planning you can make an exact duplicate of your current return and play with that. 

For online, go the tools menu and select return manager. There is a copy button under your file. Hit copy... Note, you will not be able to print anything from this file. If you wish to do an adjustment, use the original file so  you can print out any schedules that are modified.

For the Desktop/CD version, go into the forms method and select the "File" > save as> It will give you the exact same name with (1) behind it. You can play with this without changing the info in the original file.