Replacing Fencing with Gaps Because of Erosion
Go ahead and purchase the longer fence boards but do not bury the lower ends in the ground. The boards will just rot faster if you poke then into the soil.  Instead cut each board to length as the fence is being built. 

You can stretch a string between the posts for each section to be used to guide the top of the boards for that section. Then when you are ready for each board set it in place just resting on the ground and mark its length with a carpenter pencil at the string. (Note that if the fence boards are the dog eared style turn it upside down for marking so that you are cutting off the bottom).