Type M vs Type L Copper Pipe in Residential

[Benjamin Cohen]

I'm currently in the process of a flip in Chicago, IL and I've had an inspector tell me my plumbing was incorrect and needed to be revisited. After bringing a more reputable plumber in he mentioned that all my rigid copper piping had red lettering (type M - lighter duty) and Chicago required blue lettering (type L - medium grade) throughout. This is likely to carry some significant costs (ripping all the copper out, buying new + labor. Has anyone heard this before? I haven't seen this in other states, so I assume it's a Chicago thing.