I have two separate drawing one of a 2'x2' ceiling plan grids which i created myself and another which was submitted by the AC company i have to merge these two drawings.And i have to move the ac supply vents and the return vents to occupy a 2'x2' grid space so that they would not be in conflict with the proposed locations of the also 2'x2' recessed lighting.Any advice on how to carry out this task quickly and efficiently. 

I think XREF is your friend here.  If you start copying drawings into other drawings you're going to run into a mess in a hurry.Dave.