#1
I bought a righty 12 string a few months ago, and Im left handed so Ive just been playing it upside down. But I want to restring it so I can play lefty, and since the bridge on the guitar is angled, I was wondering if restringing it will effect the guitar in a negative way. Should I just restring it, or find a guitar tech to put a new bridge on it?
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#2
Defenitely. The bridge piece is slanted to accommodate for different string weight/gauche ratios that make for different stretching characteristics. By making the scale of the thinner strings a bit shorter and that of the thicker strings longer, all strings get about the same tonal intervals over the frets. Reversing the strings would then completely ruïn the intonation. A 12 string would suffer even more, because they are usually not that perfectly tempered to start with.
It's not a problem for your guitar though. It will take such abuse without adverse effects, but you and your audience will suffer badly. Having the bridge replaced is therefore necessary.

And of course the nut should be replaced as well. Your luthier will know all about it.
#3
^ He's right, even though it's kinda too technical a way of putting it.
The slant is referred to as a "compensated" bridge. The angle allows for closer intonation because of the thickness changes of the strings.
Classicals don't have much(if any) compensation to their bridge saddles because the strings are all so uniform in thickness. My piece of strat clone has massive amounts of it(low E is about an inch farther away from the neck than the high E saddle is).
If you're curious as to what all goes into changing a righty over to a lefty, read on at the following link:

http://www.frets.com/FRETSPages/Musician/Guitar/Setup/LeftConv/leftconv.html
#4
Thanks to both of you for the replies, I appreciate it.
The softest of blankets is six feet of soil.

Upcoming Shows:
Sasquatch! Festival - May 2013