If he’s guilty then there’s no harm in doing an additional DNA test to confirm this.
I’d argue that there can be harm and a grave miscarriage of justice, depending on what is being tested.
Testing new samples from Cooper to confirm that the samples used to convict him, were really his DNA, seems fine. If nefarious racist plotters faked his DNA in 1983, I guess new samples wouldn’t match his 1983 lab results.
Retesting 25-year-old samples from the crime scenes, has some likelihood of producing meaningless “no match” results because the samples have degraded over time, possibly worsened by storage conditions. Retesting the crime scene evidence would be nothing but a scam to pretend it shows “innocence”, when it does nothing of the kind.