I don't think the recent change of oil has anything to do with the rpm behavior. While it is normal for the engine to have up to 1500rpm while warming up in the morning, it is strange if the rpm normalizes to 750-800rpm and then goes back to 1200rpm while driving. You may want a mechanic to check and clean the IACV and MAF sensor.

By the way, have you recently removed the thermostat from the engine? The one that regulates the flow of coolant?