The $$ depends I suppose on your fare class, but if your flying with a non-refundable discounted cheap fare I think it's gonna cost you the $100 (+ any price difference in the actual fares) to make the flight switch whether you switch the leg from landing in LGA to EWR or any other airport. The ffact that "they're really close to each other and in the same city area" I don't think will affect the process at all.

Call continental and ask .......

((ps ---if I'm wrong I'm sure someone smarter will post very soon))