Yes, Canon did make changes in the design and construction material of many parts in the A series. As you mention, the battery door is one change. With the introduction of the Canon AE-1 in 1976, the battery door was made of metal but by the third year of production, in 1978, changed to plastic. The A-1 was first produced in 1978 so it, along with the AE1-Program always had plastic battery doors. The AE-1 Program also had a change from metal to plastic in the rewind shaft bushing around the middle of the year production in 1983.