Yes, it is true that you can move EVERYTHING back and forth between the various statuses, however, if you have SOME of your cards in one status and some in another, you will not be able to REVERT back to "some and some" without doing so card by card or at best, set by set. There is no overarching "Move this portion of my collection from mixed sets back to the status they were". Hence the warning.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).