My LBS typically wouldn't charge for the swap if you a) bought the replacement from them, b) brought in the wheel (already removed from the bike) and c) came in at a non-busy time. Obviously if the old freewheel proves labor intensive to remove then all bets are off. Now, bring in a freewheel you bought online and ask for a swap and they'll likely charge you more than the cost of the freewheel.