Big bang or piece meal approach – what works best?
I like the piece meal approach. With this approach, you can "pilot" a group and allow them to find the issues.
To me the word "piecemeal" could be replaced by "organic" or even "agile". I've been involved with enough big projects that attempt to predict what will be needed by the time they complete and are so far off the mark that their cost is just money down the drain.
That's not to say that an occasional "radical shift" is't justified, but I am of the opinion that organic change give more chance to ensure that one is heading in the right direction.
It is always better to do "incremental" and "iterative" approaches - you learn and improve as you go.