If you enjoy passing people on two lane highways or merging into traffic easily this is a terrible idea. In the US the highway system and traffic laws are all designed with consideration to the driver being on the left side of the vehicle. It's nice to be different but there is a point at which it is so impractical that it's not worth it.
If you do this I think you will immediately regret it.
To answer your actual question, it would likely be a crapload of work and a lot of money in parts to switch the LHD to RHD, you are probably better off buying an RHD car. Should be fun to try to drive with the pedals backward and the shifter on the other side. I bet you could get a steering wheel with pedals and a shifter as a computer peripheral that is RHD so you can get used to it before you buy the car. Pick up one of those and a driving sim and you'll be all trained up before your car is in the country.
2009 STi - Cobb Stage 2