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I'm glad fake-PAE works for you.

Have you got any custom PPAs? You can check that with Synaptic (check for program sources via Settings -- Repositories and the tab Other Software). For example fake-PAE will need to be restored if wiped by the upgrade. But that one should stay (and used to stay earlier at upgrades to new versions).

So before pushing the upgrading further, I suggest that you make a good backup. Something could go wrong.

An alternative is to install directly into 13.04 (or even 13.10 alpha or beta) with Lubuntu fake-PAE according to one of these links.

One Button Installer, 'OBI'

It is a good choice unless you have a lot of tweaks and private files in your current installed system, or if you prefer Xubuntu. On the other hand, I run Lubuntu plus Xubuntu Desktop to get the best from both Lubuntu and Xubuntu.

sudo apt-get install xubuntu-desktop