Hardly a Flex-ible Journey: Moving from Flex 4-Beta 2 to a Stable Build Or Nightly Build

1. Search and replace all instances of 4.0.0.10485 with the desired version (4.0.0.13875 in my case) which hopefully has already been published to maven or you have uploaded to your local repository.
   
   1. com.adobe.flex:flex-framework and com.adobe.flex.framework:compiler were updated for me
2. Change the version of your org.sonatype.flexmojos:flexmojos-maven-plugin to be 3.6-SNAPSHOT
   
   1. Usually the repository for this is the following one: http://repository.sonatype.org/content/groups/flexgroup/
   2. Make DAMN sure that in your repository definition you have the snapshots enabled set to true
3. Some of the locale related SWC files are not published on the maven repositories but you can overcome that by downloading the SDK and uploading the missing files manually to your local maven repository.
   
4. Don't forget to tell your Flash Builder IDE about the SDK ... Project > Properties > Flex Compiler > Use a specific SDK: Flex 4.0 (build 13875)
   
5. Replace all instances of xmlns:mx="library://ns.adobe.com/flex/halo" with xmlns:ns="library://ns.adobe.com/flex/mx" using your favorite tool, mine is pspad.
   
6. Replace all instances of mx: with ns:
7. Follow the instructions from this post to get your code compiling. Note: I said compiling ... not functioning!
8. Update your injection libraries if you use any. I use Parsley so I went and downloaded parsley-2.2.0 and YES if you use maven then you also need to update the parsley version in pom.xml in order to use it.

How to setup a Sandbox for Flex 4 SDKs

1) Unzip the SDKs, as you download them,into the recommended directory structure shown here:
C:\apps\FlashBuilderBeta2\sdks\3.4.1
C:\apps\FlashBuilderBeta2\sdks\4.0.0
C:\apps\FlashBuilderBeta2\sdks\4.0.0.13875

2) In Flash Builder 4, add the SDK by goign to: Window > Preferences > Flash Builder > Installed Flex SDKs. Don't make it the default if you want the older stuff to still function.

3) Create you sandbox project and be sure to specifically select the newly added SDK, rather than the default.

3) Add this compiler option in your project settings (right-click on
your project-->properties-->Flex Compiler-->under additional compiler
arguments):
-use-network=false

4) You may also need to make the following modification in Flash Builder: Project->Properties->Build Path->Library Path->Framework Linkage and select 'Merged into code'.

5) Also one very important action which you must perform as pointed out here:
1) Rename the folder /frameworks/libs/player/10.0 to "10"
2) Edit the file /frameworks/flex-config.xml and remove ".{targetPlayerMinorVersion}" and save

6) If you still have problems, here are some references that might help you:
Reference A
Reference B