Normally, you will want to fly your overhead icon over your fields at the start of every year so that the costs for the year get passed on to that year's crops. Right now you can do one of the following:
Go back and refly the fields using the same December '96 date. Only this time switch the acres to a negative number. This will reverse the previous action. Then go to Allocate Inputs (under the Enterprise menu) and allocate the overhead (under equipment) to your 1996 crops (you can use the Show Me button to show only 96 crop enterprises). Then after you close your 96 year, you should fly the fields with a January date for 1997 costs. This will give you 96 overhead allocated to 96 crops and 97 overhead to 97 crops.
The alternative is to do exactly as you said and leave the 96 overhead allocated to 97 crops and refly the fields at the end of 97. Again, the only drawback will be that each year's costs will go to next year's crops.