What car are you proposing on changing?
Some, like the Integrale and E30 M3 and worth slightly less in RHD format due to the conversions tampering with the "purity" of the original design.
For a "normal" car like the GTA I would leave it in LHD.
For the 8C, that would cost a fortune and, unless you did it properly, would likely devalue the car.
Some cars are worth a bit more if converted to RHD, however. But that depends on who you are and who you get to do it.
For example, if you are the Sultan of Brunei and you commission Pininfarina to convert some Ferrari F40s to RHD these will be worth more than the standard cars:
Ferrari F40
"Production was discontinued in 1992 after 1311 examples had been completed in five years, but while the F40 was only ever available from Ferrari in left-hand drive, a handful of very special cars were re-finished in right-hand drive by Pininfarina for the Sultan of Brunei. Constructed for this most avid collector at vast expense, relatively little is know about these intriguing Speciale's, only one car having made it out of captivity. All received custom-fitted leather interiors, most using Testarossa seats, instrument binnaces and door trim. New bodywork shades were used, these including Grigio, Blu and Giallo Fly."
