That depends on both how you read Tarot cards and how you use reversals. There are many ways to read Tarot. We all seem to have to find our own paths.
In the way I read, it's possible that the upright Sun could have a negative meaning. I read very intuitively, simply by analysing the card's imagery in conjunction with the question asked. I rarely use books or book meanings, if I ever do check the book meanings it's just to flesh out some ideas, not a major part of the reading at all.
Now the thing about reading intuitively is that not only does the same card in different decks have different meanings, but also the exact same card in the same deck can and usually does mean very very diffferent things on different occasions. So yes, one day a card can have a positive message, the next day the very same card can have a negative one. I use the word negative here loosely, as nothing is "negative" in Tarot, it is a great tool to improve ones life. All it's messages are useful and usefulness in our lives is a positive thing.
The way I use reversals would not change a positive card into a negative one though. In the method I use, a reversed card has the exact same meaning as the upright one, BUT the reversla indicates a blockage that needs to be corrected. The reversed card indicates the same energy is active as in the upright card, this energy or event is trying to assert itself in your life, but you are doing something to block it. It can only manifest itself in your life once you stop blocking it. THat means that if this is an event you want to happen, you need to figure out how you are preventing it from happening and to stop doing so. If the event is something you don't wish to happen, you can do readings to see how you can prevent it...
Hope this helps,
Bar
