Running build events is not a good way to go. You're way better off having a dedicated build script to do the work for you.
Since you're on windows, checkout the use of PowerShell to run your builds... there's lots of resources out there. (personally I use PSAKE - pronounced sa-kee, as in Japanese rice wine)
Now, on to the renaming. Here's a snippet of code you can use to rename all files in a directory to the lower case equivalent.
Rename-Files.ps1
$dir = "$HOME\Projects\MyProjectDir\FilesToRename"
dir $dir | Rename-Item -NewName {$_.Name.ToLower()}
then from the console
> .\Rename-Files.ps1
note: don't forget to elevate powershell permissions