I upgraded from elements 10 to 13 today. When PSE 13 was installed, I uninstalled PSE 10. I also own Lightroom 4.4. However, I used to be able to switch from LR4, to PSE 10 now I cannot switch to PSE 13. (Photo>Edit In>Edit in Photoshop) is grey. Why?

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What I have found is that when I upgraded from PSE12 to 13, my Lightroom 5.6 still selected PSE12 in the list of external editors. I had to use the preferences menu to select the right PhotoshopElementsEditor.exe in the Program Files/Adobe/Photoshop Elements 13 folder. I got a message stating that Photoshop Elements was already selected as external editor. In that message box, there is a button with the option to 'change anyway', which I did. Now LR correctly opens PSE13 when I choose Edit in Elements.