Do not rename SRB2.srb to SRB2.wad. It won't affect the SRB2, however if you host a netgame everyone playing will be required to change their SRB2.srb into SRB2.wad, and you'll also have to change the extention if you want to join a netgame. SRB2 won't let you join an SRB2.srb netgame if you have SRB2.wad. Instead, save yourself the trouble by copying the file, pasting it in the Doom Builder directory, and then renaming the extention to .wad. That way you can join and host netgames and use Doom Builder.