Off the top of my head after spending time thinking about the game as a whole and not merely the graphics....
Simple mirror-image versions are not going to work in all cases. Avatars with logos for example the Lexus one:
Now the logo for the lefty is bassackwards! There's a few more including Ketel & GolfChannel NBC that would also need added artistic work. Perhaps even some of the country flag avatars would need added work also.
WGT also needs to do the work to mirror all the clubs. The images of clubs in the club selection area as well will need artist's work.
Have you considered the work to account for missed dings? If a righty misses late the ball goes to the right, if a lefty misses late the ball should go left. There's more work to be done in the guts of the game.
Potential work in uneven lies flipping sidehill lies. What's downhill to a righty is uphill to a lefty.
Potential work in future features "stance adjustment" and "fade/draw." (I got a inkling this stuff for rightys might already be somewhat implemented from early versions of the game.)
Looking at the game window with the mini-map on the right of the screen makes me wonder, based on the limited camera angles, if a lefty avatar is now much more likely to be hidden by the mini-map. Potential work to be done re-positioning the mini-map opposite the lefty avatar.
Are there performance impacts? I was in an AS game yesterday where 3 of the 4 players had the same avatar. That means less avatar files to download. With lefty avatars righty/lefty won't be able to share files. Will there more often be additional avatar files to download?
How about GUI impacts? We better know what is acceptable to the users first. Does everyone want the option of playing lefty? We're all ambidextrous then. Or do you sign up as a lefty and are then stuck with it? Which GUI needs to change? registration or game options or appearance options?
Are there Pro Shop impacts? Does WGT sell lefty avatars separately? When you buy an avatar do you specify left/right?
How about Lefty only tournaments? Should that be accommodated in the tournaments features?
I think I've made my point. There's going to be much more work in the entire game than just flipping the avatar pictures. That's just the tip of the iceberg IMO.
Certainly a more detailed feature definition, a feature specification would be needed before any high level estimate could be given with any degree of certainty. In addiiton, without knowing AS3, or whether the original designer is even still around (we both know maintaining/enhancing someone else's stuff is a PITA) I'd be stupid to say it was a simple change.