if you do that more than three times in one week the sec (?) can fine you and/or restrict your trading abilities.
Usually, when I sell one stock to get into another one the same day, nothing is said if it is just once a week kind of thing.
Basically if you sell a stock it takes 2-3 days for the $$ to clear, so when you sell and buy prior to the funds clearing from the sale, you are buying with $$ that actually aren't in your account yet. and this is why you have to click through the warning scottrade gives you.
However I have had no issues if I do this every now and then.