Why would you want to do that? IMHO, the desktop software is a better product.
They are not interchangeable. They are 2 different products. Your CD includes a state program, for instance, while in the online product, preparation of a state return is an extra charge. Your desktop software also has the popular Forms Mode which the Online version does not have. This allows for direct viewing/printing of forms during return preparation and even editing on the forms in problem situations.
You can also prepare multiple returns in your desktop software, and do "what-if" scenarios when tax planning. You can efile up to 5 Federal tax returns (if more than 5, the additional ones can be printed and mailed). You can prepare multiple returns of your included state, but there is a 24.99 fee to efile each state return (no state efile fee for New York state, and no fee to print and mail any state returns.)
