Is it hard or is it same thing to go between two i might want to switch them up and just want to know.
no it isn't hard at all, just make sure that the drivers of the old card are completely removed before installing the new one. Also depending on what your upgrading to make sure your PSU can handle it
If you leave drivers from ATi installed when you put in your nVidia card, things will go crazy. Run Driver Cleaner in safe mode to get rid of Catalyst first, then install your nV card and Forceware.
You can run without even removing the ATI drivers but it's certainly best to remove them before you take out the card. ATI's recent software is good at removing itself, check the instructions on the website under driver downloads. I don't think switching between ATI and nVidia (or vice versa) is any more troublesome than switching one ATI card for another (or similarly switching nVidia for nVidia).