I upgraded my NVIDIA drivers without any problem on Ubuntu.
Version: 295.33 Certified
Release Date: 2012.03.22
Operating System: Linux 64-bit

Just downloaded the NVIDIA drivers and installed without any issue:

cd xxxxxxxx
sh NVIDIA....pkg.run

Without exiting X and going through GRUB...?

I got a black screen when I tried!

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I explain step by step what I did.

Before trying to update anything I updated the system. I got a black screen because the kernel was upgraded. At this step I wasn't able to enter X ( I understand that this was because the drivers are compiled during the instalation).

So I reinstaled the drivers I had before from the console:
Go to the path you downloaded the NVIDIA drivers
cd xxxxxxxx
Install NVIDIA drivers
sh NVIDIA....pkg.run
Answer yes to all questions from the installer.

Entered X with startx
I downladed the new drivers from NVIDIA

Then instaled the new version.
cd xxxxxxxx
Install NVIDIA drivers
sh NVIDIA....pkg.run

It worked fine at first.

I upgraded my NVIDIA drivers without any problem on Ubuntu.
Version: 295.40 Certified
Release Date: 2012.04.11
Operating System: Linux 64-bit

Just downloaded the NVIDIA drivers and installed without any issue:

cd xxxxxxxx
sh NVIDIA....pkg.run