In theory, you're absolutely right - there is no good reason why a leftie shouldn't be able to adapt like any right sided player who plays on the left (KK and Carlos being two good examples)
In reality, I guess it's about familiarity and opportunity. As right footers are more common, it's more likely they'll have at least played on the other side at some point through their development, and will be familiar with the adaptations they need to make when playing the role on the other side.
As left footer are a rarer breed, they generally play on the left and nowhere else - and may not have ever had to adapt their game to their other foot. They may not be able to make that transition and be as effective on the other side of the pitch.
And it's that unknown element which makes a big money transfer a risk most wouldn't want to take when the intention is to play them out of position from the off.