Remember, though:
If you have used ANY days already on your 4-park Flexticket, these days transfer onto your 5-park Flexticket.
In other words, if you started using your 4-park pass on Sunday, and decide to upgrade on Wednesday, the "time frame" for your new 5-park Flexticket will also begin on Sunday (not the day you upgrade) - so you don't get an additional 10 days.
Of course, if you haven't used your 4-park Flextickets yet - don't worry about a thing (other than the price of upgrading).
Any of the theme park Guest Service departments can do this transaction, including Sea World and Busch Gardens