To create a complete new biochemistry, we are going to make carbon-carbon bonds obsolete. Carbon-carbon bonds are the backbone of our carbon-based life. To make them obsolete, we will force life to resort to some other basic bonds.
Radiation in the wavelength of 330-350 nm will be absorbed by carbon-carbon bonds, and if enough energy is provided, it will break the bond. If I am not completely off, Si-Si bonds absorb chiefly in the range around 460nm.
Nitrogen-gas lasers spark in the 340nm range, which is exactly what we would need. Now, on Earth there is a lot of C-C bonds, so the energy needed to succeed will have to be rather large, and the dosage will need to occur over a longer period of time.
To achieve this, we are going to add a new satellite to the Earth. This new celestial body is heavy and covered in a sufficiently dense atmosphere of nitrogen. The body itself is made of piezoelectric material and, not being tidally locked to the Earth, it suffers constant tidal forces, which translate in huge sparks across the nitrogen atmosphere, resulting in a constant, Moon-size, UV laser pointed at the Earth.
The new satellite, Moon2, also carries a smaller set of its own satellites, all made of solid CFC. When Moon2 is captured by the Earth, its satellites fall onto our planet, burning in the atmosphere and releasing massive doses of gaseous CFC, which get rid of the ozone layer, and make our Nitrogen laser way more effective.
Let Moon2 in orbit long enough, and carbon-based life may become less-likely than silicon-based life. The massive doses of CFC may also poison the larger life-forms, which stood a better chance of surviving the radiation, or actively find shelter.
Some pockets of carbon-based life may remain, hid in caves, or deep at the bottom of the seas, but as time passes, they will become just niche-survivors, as rare as a Dodo today.
Finally, let the Moon and Moon2 collide together before waking up the humans, and welcome them back with a huge firework.