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Hi,

To say true: I cannot find an answer to your question. Theoretically Autodesk should provide new installation image with all Service packs integrated (SP1, SP2 and SP3) to download for those users who already have Yosemite installed on their Macs and need a fresh installation of AutoCAD 2013 for some case.
There was already such a case with SP1 for AutoCAD 2013 for Mac and OSX 10.8 (Mountain Lion) and special installation image was released for AutoCAD 2013 with SP1 integrated in it.

Looks like at the moment there is no such installation image for Yosemite on Autodesk servers, so your question should be addressed to some from Autodesk stuff. Let's wait for the answer from them.

Maxim