Keep the 2.4L. With all the extras needs to complete the swap it would cost more than the car is worth. There also isn't a big difference in power, as the V6 only has 20 more HP and lb/ft of torque: 2.4L(162/164), 2.7L(182/182).
I agree with spongebob, total waste of time and money and really no advantage. Also the V6 uses a timing belt and the 2.4L uses a chain. Better mileage on the 2.4L too. Also the V6 is a very old engine and will be gone from production when the current 2012 model has run its coarse.
I had the 2.4L in a '08 Rondo and what a great engine, lots of power and very good gas mileage, I had no interest in the V6 at the time of purchase, just did not see any advantage for the extra cost.
They did tweak the newer 2.4L and now its rated at 175 hp.
Additional thought - you can kiss goodbye to any value in your Rondo both to sell and trade, when its found out its not the original engine built with the vehicle as it indicates that in your VIN code.