I have amended individual returns within the 3 year time frame and changed Sec179 depreciation. 

A taxpayer may make, revoke, or change an election without IRS consent on an amended return  per code sec 179(c)(2);  Reg 1.179-5(c); Rev Proc 2017-33 

Does this apply to S Corporations - I don't know      Just giving you information that may help lead you to the answer you need.