Try on the Advanced Mode menu of the Intel(R) Graphics Media Accelerator control panel. Go to Media > Image Enhancement and disable "Adaptive Contrast Enhancement" (if available)
Also go to Power Options, change the Power source to "On Battery" and disable "Display Power Saving Technology".
You may also try the troubleshooting information that is available on this URL:
If the problem continues you should check directly with DELL to see what could have changed after that hardware replacement.
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I had the exact same problem with the exact same computer (never had the screen replaced though). And for many frustrating months I thought the same thing: that it was caused by the "Display Power Saving Technology" but it turns out this isn't the case. The culprit is actually a stupid, hidden Dell option called "Dell Intelligent Display." For whatever reason, the only way to access this option is to turn off Secure Boot Mode. Here's what you do:
Restart your computer and hit F2 when the Dell logo comes up.
Go to the "Boot" tab and disable Secure Boot.
Save and exit and continue with the reboot.
When it's up and running, go to your desktop, right click on the battery icon and select "Power Options."
On the left side of the Power Options window, click on "Dell Extended Battery Life Options."
In the first tab of the new window, you should now see the "Dell Intelligent Display" option which can be disabled.
I found this solution somewhere else online and tried it today. So far, it's worked perfectly. No more obnoxious auto-adjustments to the brightness, whether I'm plugged in or not. I have no idea why Dell thought it was a good idea to squirrel away this option and hide it from users. I'm just happy to have it fixed. I really like this computer but the constant brightness/contrast adjustment issues were really getting to me. Hopefully it will work for you as well.
Oh and for good measure I also have kept both the "Adaptive Contrast Enhancement" and "Display Power Saving Technology" turned off.