If I remember the AoE Expansion...
To play them: Yes, of course.The scenarios won't be as good compared too other ones than now as they won't use the new features, of course, but you will be able to play them with the new units, technologies and all other features.
To edit them:It is very probably that this will also be possible.And if not, it is still no problem,well, at least for the map, you could just open the old scenario, select the whole map with the copy function, copy it and then insert it in a new map (in the new format).But to copy all the triggers could take a huge amount of time if there are hundreds - still much fewer time than doing everything again.But I think AoK will be able to do this for the designer then, as it should be really not to hard to program this.
I'm sorry about my bad English.