D3: Update topojson map

Question

I am stuck trying to update my topoJSON map in D3. I have a map of the counties of a state (Maryland) in the US. I have two sets of GeoJSON data:

us.json - population of the counties in 2012

us05.json - population in 2005

I want to:

1. build the map of the counties (works)

2. map out circles relative to population size of each county for 2013 on button click (works)

3. map out circle ... for 2005 on button click (works)

4. update map between button clicks (does NOT work)

I was going through http://bl.ocks.org/mbostock/3808234 so I made my update, enter, and exit colors the same. But, I am getting stuck on GREEN - as in the enter color. So my circles are not getting past enter?

Build the map:

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

d3.json("us.json", function(error, md) {
  if (error) return console.error(error);

  // states  
  svg.append("path")
      .datum(topojson.mesh(md, md.objects.states, function(a, b) {
          return a !== b; }))
      .attr("class", "border border--state")
      .attr("d", path);
});

Call update2012 or update2005 depending on button click:

function update2012() { 
d3.json("us.json", function(error, md) {
  if (error) return console.error(error);

  var data = topojson.feature(md, md.objects.counties).features
        .sort(function(a, b) { 
            return b.properties.population - a.properties.population; });

  update(data); 


});
}

function update2005() {
    d3.json("us05.json", function(error, md) {
      if (error) return console.error(error);

      var data = topojson.feature(md, md.objects.counties).features
            .sort(function(a, b) { 
                return b.properties.population - a.properties.population; });

      update(data);
  });
}

Update():

function update(data) { 

// new data joins old elements 'circle'
var update = svg.append("g")
    .selectAll("circle")
    .data(data);

// update 
update.attr("class", "update")
    .transition().duration(2000)
        .attr("transform", function(d) { 
            return "translate(" + path.centroid(d) + ")"; })
            .attr("r", 5.0);

// enter new elements 
update.enter()
    .append("svg:circle").attr("class", "enter")
      .attr("transform", function(d) { return "translate(" + path.centroid(d) + ")"; })
      .attr("r", 0.0)
      .transition().duration(1500)
        .attr("r", function(d) { 
            return radius(d.properties.population) * 2; });

// exit - remove old elements 
update.exit()
    .attr("class", "exit")
    .transition().duration(2000)
        .attr("r", 0.0)
        .style("fill-opacity", 0.01)
        .remove();
}  

There is code redundancy. I call d3.json() so many times because I want to build the map first and then either the 2005 or 2012 data depending on the button clicked. I am attempting to create green circles, switching to gray, and then fading away as brown. Instead, when I click on a button, they transition properly but stay as green. When I click the other button the new circles are constructed over / within the old ones and everything is green.

How do you transition and update between these topoJSON maps? Keep in mind, the topoJSON data has all of the same properties between us.json and us05.json EXCEPT the population size. I'm sure I could be combining the two datasets, but let's tackle one problem at a time!