Well, I don't think it's risky... just indecent.
First, you don't know where the signature is inside the file, in fact. Suppose that avast announces a virus there... but how would you find out where did avast spot the signature?
Second, no matter if the signature is encrypted or not, it's not that hard to find out what given antivirus is looking for. Modifying the malware is just a creation of new variant... similar to packing by some strange packer etc. - i.e. nothing new, in fact.