diff --git a/lib/vagrant/plugin/v1/config.rb b/lib/vagrant/plugin/v1/config.rb
index 5a6e7f309..7d5e8b243 100644
--- a/lib/vagrant/plugin/v1/config.rb
+++ b/lib/vagrant/plugin/v1/config.rb
@@ -5,6 +5,13 @@ module Vagrant
       # V1. Any configuration key plugins for V1 should inherit from this
       # class.
       class Config
+        # This constant represents an unset value. This is useful so it is
+        # possible to know the difference between a configuration value that
+        # was never set, and a value that is nil (explicitly). Best practice
+        # is to initialize all variables to this value, then the {#merge}
+        # method below will "just work" in many cases.
+        UNSET_VALUE = Object.new
+
         # This is called as a last-minute hook that allows the configuration
         # object to finalize itself before it will be put into use. This is
         # a useful place to do some defaults in the case the user didn't
diff --git a/plugins/kernel_v1/config/nfs.rb b/plugins/kernel_v1/config/nfs.rb
index 56041ba1a..2faae956a 100644
--- a/plugins/kernel_v1/config/nfs.rb
+++ b/plugins/kernel_v1/config/nfs.rb
@@ -5,6 +5,16 @@ module VagrantPlugins
     class NFSConfig < Vagrant.plugin("1", :config)
       attr_accessor :map_uid
       attr_accessor :map_gid
+
+      def initialize
+        @map_uid = UNSET_VALUE
+        @map_gid = UNSET_VALUE
+      end
+
+      def upgrade(new)
+        new.nfs.map_uid = @map_uid if @map_uid != UNSET_VALUE
+        new.nfs.map_gid = @map_gid if @map_gid != UNSET_VALUE
+      end
     end
   end
 end
diff --git a/plugins/kernel_v1/config/package.rb b/plugins/kernel_v1/config/package.rb
index 0f4c1d235..3b521d785 100644
--- a/plugins/kernel_v1/config/package.rb
+++ b/plugins/kernel_v1/config/package.rb
@@ -4,6 +4,14 @@ module VagrantPlugins
   module Kernel_V1
     class PackageConfig < Vagrant.plugin("1", :config)
       attr_accessor :name
+
+      def initialize
+        @name = UNSET_VALUE
+      end
+
+      def upgrade(new)
+        new.package.name = @name if @name != UNSET_VALUE
+      end
     end
   end
 end
diff --git a/plugins/kernel_v1/config/ssh.rb b/plugins/kernel_v1/config/ssh.rb
index d6719a105..d6fdf159f 100644
--- a/plugins/kernel_v1/config/ssh.rb
+++ b/plugins/kernel_v1/config/ssh.rb
@@ -15,15 +15,32 @@ module VagrantPlugins
       attr_accessor :forward_x11
       attr_accessor :shell
 
-      def validate(env, errors)
-        [:username, :host, :max_tries, :timeout].each do |field|
-          value = instance_variable_get("@#{field}".to_sym)
-          errors.add(I18n.t("vagrant.config.common.error_empty", :field => field)) if !value
-        end
+      def initialize
+        @username         = UNSET_VALUE
+        @password         = UNSET_VALUE
+        @host             = UNSET_VALUE
+        @port             = UNSET_VALUE
+        @guest_port       = UNSET_VALUE
+        @max_tries        = UNSET_VALUE
+        @timeout          = UNSET_VALUE
+        @private_key_path = UNSET_VALUE
+        @forward_agent    = UNSET_VALUE
+        @forward_x11      = UNSET_VALUE
+        @shell            = UNSET_VALUE
+      end
 
-        if private_key_path && !File.file?(File.expand_path(private_key_path, env.root_path))
-          errors.add(I18n.t("vagrant.config.ssh.private_key_missing", :path => private_key_path))
-        end
+      def upgrade(new)
+        new.ssh.username         = @username if @username != UNSET_VALUE
+        new.ssh.password         = @password if @password != UNSET_VALUE
+        new.ssh.host             = @host if @host != UNSET_VALUE
+        new.ssh.port             = @port if @port != UNSET_VALUE
+        new.ssh.guest_port       = @guest_port if @guest_port != UNSET_VALUE
+        new.ssh.max_tries        = @max_tries if @max_tries != UNSET_VALUE
+        new.ssh.timeout          = @timeout if @timeout != UNSET_VALUE
+        new.ssh.private_key_path = @private_key_path if @private_key_path != UNSET_VALUE
+        new.ssh.forward_agent    = @forward_agent if @forward_agent != UNSET_VALUE
+        new.ssh.forward_x11      = @forward_x11 if @forward_x11 != UNSET_VALUE
+        new.ssh.shell            = @shell if @shell != UNSET_VALUE
       end
     end
   end
diff --git a/plugins/kernel_v1/config/vagrant.rb b/plugins/kernel_v1/config/vagrant.rb
index ed3eee2c7..b9730a056 100644
--- a/plugins/kernel_v1/config/vagrant.rb
+++ b/plugins/kernel_v1/config/vagrant.rb
@@ -5,6 +5,16 @@ module VagrantPlugins
     class VagrantConfig < Vagrant.plugin("1", :config)
       attr_accessor :dotfile_name
       attr_accessor :host
+
+      def initialize
+        @dotfile_name = UNSET_VALUE
+        @host         = UNSET_VALUE
+      end
+
+      def upgrade(new)
+        new.vagrant.dotfile_name = @dotfile_name if @dotfile_name != UNSET_VALUE
+        new.vagrant.host = @host if @host != UNSET_VALUE
+      end
     end
   end
 end
diff --git a/test/unit/vagrant/plugin/v1/config_test.rb b/test/unit/vagrant/plugin/v1/config_test.rb
index 32e08c518..425984794 100644
--- a/test/unit/vagrant/plugin/v1/config_test.rb
+++ b/test/unit/vagrant/plugin/v1/config_test.rb
@@ -10,6 +10,12 @@ describe Vagrant::Plugin::V1::Config do
     end
   end
 
+  it "has an UNSET_VALUE constant" do
+    value = described_class.const_get("UNSET_VALUE")
+    value.should be_kind_of Object
+    value.should eql(described_class.const_get("UNSET_VALUE"))
+  end
+
   describe "merging" do
     it "should merge by default by simply copying each instance variable" do
       one = foo_class.new