It is always better to purchase the correct guitar to suit whether you are left or right handed. However is not always possible to buy the guitar you love in a left handed version.
In order to basically convert an acoustic guitar a couple of things must be changed. The NUT must be replaced as the string slots or groves will be too small to accommodate the bass strings and too large for the treble strings. Simply cutting the small slots wider for the bass strings will still leave the treble strings to vibrate freely in the large slots which not only looks bad, it will cause buzzing. Due to the shape of most nuts, it is not possible to simply flip them around.
Remember the angle of the saddle slot on an acoustic guitar determines string length and this has a direct effect on the guitars intonation (its ability to play in tune in all areas of the neck). In order to correct the intonation and reverse the angle of the saddle, the slots must be filled and pre-cut, or the bridge completely replaced.
It is not advisable to remove and change pickguards; this will cause marks left behind as this is usually bonded/glued.
All this work above can be done by a good professional luthier; they will be very used to completing this guitar conversion.
Often on an electric guitar, hardware like tailpieces can simply be replaced with their lefty equivalents; therefore this is a much easier job to complete at home.
If you are purchasing a new guitar, it is always recommended that you order the left handed version if available; still many companies do not offer left handed guitars. Generally converting a guitar to left handed presents no structural or tonal difficulties, or damage. There are a few other things to consider; the side markers along the neck would be on the wrong side. They can always be installed on the proper side if this is an issue for the player; plus an extra pickguard can also be installed on the correct side. Luckily many guitars are made without pickguards.