Assembly Bottom Up & Top Down

I have different part files and subassemblies, all combined into a bottom-up assembly. Now, I would like to create a top-down assembly. Is there an easy way to do that? If so, how to do it? Also, which type of assembly (Top Down or Bottom Up) is better for creating a “Master Design Table” for an assembly, where the dimensions of all the parts can be controlled from that design table?