Southpaw Playing Right

Jimi Hendrix, Paul McCartney, Kurt Cobain, these are the faces people think of left handed guitarists are mentioned. For Hendrix and McCartney who became famous in a time before southpaw guitars were made, the tales of how they flipped the strings on right handed guitars and played it up side down have become legend.

Guitars have come along way since that era, which make it impossible to flip strings. This is a nut:

The strings go though the holes, and as you can see there is no way in hell you're getting the G string [The thickest string on a four string bass] into the hole meant for the E string. You will be able to get the E string into G slot but it's going to move around alot and you're just not going to get the sound that guitar was meant to make. 

You could go to a guitar repair shop and have them trade out for a left handed nut and bridge, [yeah bridge, that thing that keeps your strings attached at the bottom, you'll need a left handed one of those too] but that's going to add to the cost and kill the resale value. 

Even if you do all that, you'll still be playing your bass up side down. That don't seem so bad until you think if like using a microwave up side down. All the mechanics will be hanging a way they're aren't supposed to, [the only way to have the spinning plate is to glue it to what is now the top or not have it at all] you just won't have the same quality instrument you started out with.

[Please, consider becoming a luthier if you really want to mess with your guitar.]