i remember like 12 years ago when i used to fiddle around things like those …
in most cases i only had to hex edit the copyright, author and some few words in an exe to make it undetectable …
things got a bit harder later on, but that might still work for the new viruses that just got added into the av library.
also not that decompressing/compressing viruses with upx or such programs, adding some sort of protection may disable almost any connectivity that the backdoor can do, starting from sending an email and ending with direct/reverse connect.
in some rare cases it can send over lan but nothing more ….