a reason for the change over could well have been that the lunar calander is not as reliable as the solar calander in that it shifts alot accross the seasons. I remember my friend doing ramadam in 1993 and it was february as we know this year it was november the islamic lunar calander follows the phases of the moon closely and from that example above you can see the shift over a period of time in relation to the seasons. I don't really know enough about that though maybe some one else does.
carolinexx