Because the natural human body is entirely biological, replacing some of the biology with synthetic equivalents needs to be done with extreme care. Evolution has a habit of creating systems and parts that do more than one thing, so if we are going to dispense with some biological part, we need to be sure that we are not accidentally removing any secondary functions that the part had. An example of this is the skeleton. Bone, as well as being a structural material, acts as a mineral store. A body with a synthetic skeleton will need some other arrangement for its mineral store.
The general principle of modularisation involves clarifying and simplifying the functions of each part, so that they can be well-defined and controlled. Again, we have the problem of multiple functions being carried out by single parts, as well as multiple parts contributing to single functions. Biology is nothing if not messy!
We also need to be careful not to lose any of the advantages of redundancy that the messiness of biology often provides. If we have evolved more than one way to perform a function, there is probably a good reason for it, and we would be unwise to simplify things too much.
The concept of refactoring, applied to the Human Body 2.0, means understanding the various functions performed by any biological structures that will not be present (such as a bony skeleton), and making sure we provide alternate arrangements for them. It also means making sure that the proposed modularisation doesn't lead to a loss of flexibility or interfere with the interactions between various tissues (e.g. are there good reasons why the pancreas combines both endocrine and exocrine tissues in a single organ? It might be tempting to separate them into different modules, to keep blood glucose regulation separate from the production of digestive enzymes, which would appear to make good sense, but would this cause any problems?)
What are the requirements?
What Exists Now?
What can be done right now?
What needs doing now?
Speculations, suggestions, criticisms, etc.