I doubt that it will be your main bearings unless the crankcase filled with water after you changed the oil. Is the level correct on the dip stick? The normal culprit is moisture in a cylinder, and it doesnt take much to cause enough rust to make things not move. The best/cheapest rust breaker is a 50/50 mix of acetone and auto transmission
fluid. Let it sit for a day and try to move the crank, repeat as needed. I would recommend taking a lead hammer and make sure all of the valves will move before you start turning things. If there is moisture in a cylinder, you may have a stuck valve, which will cause a bent pushrod if you break the engine free, but have not freed up the valve. Just reread your post, and agree that removing the starter would be the first step. Then go for the liquid in the cylinder. Another trick that I have used with some success is to rent a steam cleaner and plumb it into the cooling
system, and heat and then cool the engine. The expansion and contraction of the metals will sometimes break up the rust and get it turning. Good luck with it and keep us posted as to your results.____Grant.