Question about default Wine prefix

Hi. It's been a while since last time I was here...

I noticed that, even with last 1.5 versions of Wine, the default OS original Wine prefix emulates is stil Windows XP. Maybe there's a good reason to keep it as it is, although I was thinking, as newer Windows applications (and games in my case) are being released as time passes by, wouldn't it be good to change the default emulated system to Windws Vista, 7, or eventually 8?

And this is probably something I haven't still understood. Are Windows applications affected very much by the prefix's emulated Windows version? Or is it just a matter of copying the missing dll files to the application's directory or the prefix directory?

And also, I don't remember how to change emulated Windows version in a prefix, it's been a while since last time I played with Wine... Is it just a matter of changing the setting in winecfg, or must I create an entire new prefix with the option?

Thanks for your help.