Conditional upload size in WordPress

I have already configured php.ini to allow up to 100MB of uploads per WordPress installation. But the problem now is that there is one installation in our VPS which needs about 500 MB upload. So if I set up 500MB limit in php.ini then it will alter the global settings. Is there any method by which I can change the global settings via php.ini (by setting upload size as 500MB) and then use lower values of file upload size per WordPress installation?

As I have mentioned there are about 5 wordpress installations in the VPS, so it is really hard to go with one php.ini upload size setting ??