Generally as long as the original copyright line and link are unchanged and visible in all page output, you can move them where you want them.
And you're more than welcome to add your own (eg. "mods (c) by such-and-such") line in addition to the original one. Just as long as you don't replace it, you're good.
---Jamin
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Diablo 3 Lead Designer Jay Wilson:
“The development of a Blizzard game is sometimes a long affair. This is how long it took us to be ready.”
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