A major demographic change in Africa was the rise of the Kingdom of Kongo in the Congo River basin. A rather large kingdom comprised of six separate provinces developed under the leadership of a powerful king. A thriving economy also developed, including a system of currency which used cowrie shells from the Indian Ocean as money.
The most important change was the rise of Islam in sub-Saharan Africa. It was brought to the area by caravans of merchants travelling into the area and became prominent about 1000 C.E. when the kings of Ghana converted to Islam. It is doubtful that they did so for reasons of conscience; but rather to improve trade relations with Islamic merchants. Interestingly, they did not require their subjects to convert also. The result was a form of hybrid religion which incorporated elements of Islam with African traditional religions.
Empire building of the ancient world (Hans in China, Romans in Europe, etc) declined around 400 C.E. Areas of Africa, like West Africa, had population growth that allowed for improved agricultural practices and participation in trade in distant locales.
#2 raises an excellent point. There are massive problems in treating Africa as one location, as it is such a vast territory that while demographics were increasing in one area, they could have been decreasing in another. I would strongly suggest that you revisit this question and specify what it is you want to find out so that we are able to help you. A tighter focus is definitely necessary.
This is a fascinating topic. I wonder, though, how much evidence would be available from period. Are you interested in demographics in the entire continent, or just in one specific part of Africa? I suspect you'll have an easier time tracking down information in the latter case. This might be especially true if you are interested in a large, well-established city such as Cairo or Alexandria. If you can (or allowed) to narrow the question down, we may be able to help you.