Matter at speed of light?

If matter traveling at the speed of light physically becomes light, what would happen to natural sunlight if it were slowed to less than the speed of light, theoretically?

Wrong assumption.
Relativity says it would take an infinite amount of energy to accelerate even the smallest imagineable amount of matter to the speed of light, not that the matter becomes light.
Interconversion between mass and energy has nothing to do with accelerating the mass to the speed of light, rather it is the anhilation or the mass, with the release of a corresponding amount of energy (that e=Mcsquared thing) and vice versa.

Nothing with mass can reach the speed of light

Nothing without mass like light can exist at any speed except the speed of light   Photons cannot slow down

EDIT: you got another answer that says light slows down in media. That is not technically correct. What happens is the photons get absorbed and reemitted by the atoms in the medium. Thus, the photons stop and start. The velocity between atoms is still c but the average velocity is less. It is like cars driving through a city withstoplights  the cars stop at every light and go the speed limit between lights

Actually, to the few others that answered your question above saying that light can't be slowed down, it can. That's why theres a term called the "refractive index". As light travels through different mediums such as water, it slows down (slower than the air medium), and thus causes the refraction( which is why things look slightly distorted in water). Just hold up a glass of water towards the sunlight, and voila, you're slowing down light .