It is an interesting flip of the order of operations if you will
In a typical case evidence is collected and then you get a warrant for a DNA sample once you have probably cause. In these cold cases they have the suspect's DNA from the crime scene and use it to identify the guy. Once they do that it does become easier to get warrants, I would think, but I don't think it lowers the standard. DNA is fairly rock-solid evidence. If they have your semen on a victim I think that's probably cause for a warrant to dig deeper. I don't see that as a low standard. The reason I lean towards this being OK, though needing clear rules defined, is that DNA evidence isn't circumstantial, it's pretty solid.
Now as I understand the Golden State Killer case they identified a guy who they thought was the killer. He had brothers but through detective work they eliminated the other brothers. They couldn't get a warrant for the guy's DNA though. They couldn't search his trash without permission either. So they tailed him until he left something behind, a drinking straw I think. Same thing with this guy, except he was the only son in the family they narrowed it to. This is the area that I think will need new laws/rules, what is and isn't legal in collecting evidence to test DNA vs the suspected killer.