I hear about two major approaches to introducing agile methods to organisations. Kaikaku (known in lean circles as radical change) is the brute force method of pushing all the change you want upon the organisation. You recognise this when basically the “normal” way of working is flipped on its head and a whole swarm of new practices are introduced.
Kaizen (known in lean circles as continuous improvement) is the softer, gentler approach to change, tweaking one bit here, and one bit there. Think of this as a way of introducing a single practice a time.
Now that we are clear about what is what, comes the interesting question of what is better?
You seem to be faced with two choices. A radical change that seems like a high chance of failure, but could also be a huge success, or smaller incremental change to do this.
In order to answer this question when I join a new team or organisation I like to consider how much their environment supports them in what they want to do. Those with supporting, nurturing or safe environments I would probably head towards Kaikaku, using education and building trust with senior stakeholders to establish longer term safety. You can increase safety and reduce the likelihood of failure if you can bring in a high proportion of the culture you’re looking for. I’ve seen this succeed really well when you have at least 50% of a team with lots of agile experience, something you tend to have when you hire someone like ThoughtWorks.
Work with less than 50% experience mixed support, and you start to decrease safety. At this point, I would start to look towards kaizen methods of improvement. Of course there are many levels at which you might apply kaizen and kaikaku, but consider safety levels when doing so.
Searching more on Kaikaku I’ve found this similar post from James Shore: http://jamesshore.com/Blog/Kaizen-and-Kaikaku.html