Is it it illegal to change a medical lab slip once a doctor has filled it out and signed it?

A specific fact or two would be helpful, but likely the doctor would need to sign off on changes.

It may depend on the type of changes. If it is something substantial, like a diagnosis or lab result, then yes, the doctor should approve the change. If it is a typo, then it is probably okay to change it after the doctor signs it.

It really depends on who made the change, when and why.