There's a single file on the disc that defines what "version" of Windows is it. If you look inside the sources folder on a disc, you'll see a file named "ei.cfg". This defines what version of Windows 7 the disc is for. All you need to do is create a new disc that has the ei.cfg file removed, and you'll be asked during the install which edition of Windows 7 to install. Personally, I prefer a USB key that has the ei.cfg file removed.
Note that you can't combine the 32 and 64-bit versions onto a single disc using the "simple" methods. However, with a little googling, you'll find forum posts from people that have combined both on single discs/USB keys, or have even combined Windows 7 with Server 2008 R2.
Intel i7 4790k @ stock, Asus Z97-PRO(Wi-Fi ac), 2x8GB Crucial DDR3 1600MHz, EVGA GTX 760
Samsung 950 Pro 512GB + 2TB Western Digital Black
Dell 2408WFP and Dell 2407WFP-HC for dual-24" goodness
Windows 10 64-bit