If the earth and everything on it magically changed rotation in one second, there would many changes in addition to the obvious that the sun would rise in the west. Tides would occur slightly more often since the Moon would be going in the direction opposite the rotation of the Earth. Unless Earth's revolution were also reversed, the day would become slightly shorter by about 8 minutes. This is because Earth's synodic day would change while its sidereal day of 23 hours and 56 minutes would stay the same. Also, man-made satellites in geosynchronous orbits would be going the wrong way! Most importantly, the Coriolis forces that determine climate by affecting wind and water currents would point the opposite way and there will be long nasty decades in which the Earth's ecosystems must adjust to the new climate.
If the Earth's rotation were changed gently over many days, VERY terrible things would happen. First of all, things might be fun at first since, especially at the equator, the ground will be accelerating under your feet! Whatever causes Earth's magnetic field would probably become messed up and we could be exposed to solar winds. Also, when the rotation is very small, people at the equator will feel a couple pounds/newtons heavier! This has to do with centrifugal forces normally making people lighter at the equator when the earth spins. The Earth actually has an equatorial bulge due to these centrifugal forces (it isn't a sphere!). The danger is that there would be high risk for the Earth's thin crust adjusting over its molten core so that the Earth becomes a sphere. At the very least, there would be terrible earthquakes and volcanoes as the crust becomes many millions of pounds/newtons heavier. Lastly, I would expect irregular weather to exist during this period.
If the earth were to magically start revolving in the opposite direction, there is the possibility that the seasons might go wacky. There would be no problem if the northern hemisphere was in the middle of winter and the southern hemisphere were in the middle of summer or vice versa. But, if it were spring somewhere, this place would start getting colder again! If it were fall/autumn, it would start getting warmer! Plants would have a very hard time resetting, but those that survived would know no difference in years to come.
A gradual change in revolution would be extremely difficult because there would have to be a time that the Earth is momentarily not revolving. At which time, the Sun's gravity will cause the Earth to fall towards it.