Hi,
Yes this requires customizations. You can create a feature or a console application to execute the required code.
I am attaching a sample to do so for one document library and it can be extended to all.
SPSite site = new SPSite(YourSiteUrl);
SPWeb web = site.OpenWeb();
SPDocumentLibrary library = (SPDocumentLibrary)web.Lists[Your Doc Library name];
library.DefaultItemOpen = DefaultItemOpen.Browser;
library.Update();
Do let me know if you require full code.
I hope this will help you out.
Thanks, Rahul Rashu