I had the same Issue and made some deep investigation. In my case it was odd, that my extension always shown as "Download" in the Private Gallery instead as "Installed" (the Green Hook). So I guessed there is a missing Link between the Gallery item and the installed Item
The answer is as simple as annoying:
The ID has to be "THE ID". So the Product ID (in the VSIX Configuration) is the reference which has to be in the Atom-File.
In Your case:
<Vsix>
<Id>b242c6de-d0a6-4ba1-9672-b866e36607d3</Id>
</Vsix>
As you see: Withyout your "MyVSIX..". After that it will recognize your Extension and if installed also hide the "Download" Button within the Gallery
For Update:
It's enough if you increase the < Version > Content.
For The Update Cycle (my experience):
- It checks on start of the Visual Studio (once Daily, if in Option enabled)
- It checks every time you open the "Extensions and Updates". So reopen after a save of the XML should be enough.
EDIT: (thanks for the Upvote)
Here the Pictures for better understaning (made in VS2013, but almost same for VS2012)
The ID in the VSIX File.
How it should look like in the Atom-Feedfile.
<Vsix>
<Id>d4d50729-a612-4260-9b2e-3a61ad5e2207</Id>
</Vsix>
How it should look if you correctly linked between Gallery and VSIX File.