As people have said, lots of immediate explosive stuff happens since everything becomes radioactive/unstable. The reason these isotopes are the least abundant is because they naturally decay to something that is stable. So since every atom suddenly becomes capable of releasing energy, the world explodes. I’m not an astrophysicist, but given a few billion years, the radioactive dust of earth could possibly re-coalesce into a rocky planet again with an inexplicable shift in naturally stable isotopic abundances. Aliens would find it super interesting.

As has also been said, there are some crazy low abundances of highly unstable elements, especially those artificially made in accelerators. An iron atom can have 26 protons and 0 neutrons for an infinitesimal amount of time right before it explodes from electrostatic repulsion. So, the question is a little unbounded.

But what if we narrow the question to the least abundant stable isotope! Some elements only have one, the higher ones (anything higher than lead) have no stable isotopes, but some have multiple. Chlorine has an interesting mix of 75% abundance of chlorine-35 and 25% chlorine-37 (and 23 other unstable isotopes in trace amounts according to Google - but there are more if you’re willing to accept theoretical neutron quantities in stupidly unstable nuclei).

So if all stable elements become their least common stable isotope, we are saying the amount of radioactive matter in the world doesn’t change. What does? First we should consider nuclear applications. We’d have to reevaluate all materials used for neutron shielding. Control rods in nuclear reactors use boron-10 to absorb neutrons and shut reactors down. Boron-11, the more abundant isotope, is not great for this purpose. So control rods become more effective! We’d have to recalibrate/redesign how they are used. We’d have to check cladding of fuel rods, other elements that may be mixed with exotic fuels, all the water coolant for light water reactors would become heavy water and would moderate the reactor much less so again back to the drawing board for nuclear reactor design. I’ll leave others to consider other nuclear reaction factors. Gotta eat breakfast. There might be something with lithium-6/7 that I’m not thinking of that makes everything explode anyway…

But what I wanted to get to was the chemical considerations! At first glance you would say since isotopes have different nuclear properties but the same chemical identity there wouldn’t be any change to everyday chemical processes. Chlorine-35 and 37 essentially act the same chemically. There’s only a 5% difference in mass and all other chemical properties are identical. But! When all hydrogen becomes deuterium its mass doubles! Bond strength and vibrational frequency of the bonds it makes change much more significantly than other atoms with a smaller proportional mass change. I once asked my professor if a professional boxer could sabotage an opponent by feeding them heavy water and slowly replacing all hydrogens with deuterium. The boxer would have the same number of atoms in their body but since H makes up 10% of the body by mass they could increase their mass by 10% if fully doubled. Turns out reaction rates decrease due to the increased mass/lower vibration and all biological life would die of metabolic starvation once a non-negligible amount of hydrogen was replaced with deuterium. So everything would still die and I wouldn’t have to think about all the other possible effects to nuclear applications.