Okay, I would like to know if there is a safe way of converting matter into antimatter, in the theoretical case of say, an manned expedition to a planet made of antimatter. How would I be able to turn an object or organism into antimatter without it or them annihilating, and convert them back for the voyage home? Say, for a plot point or something?
There is exactly this situation (not too scientifically handled) in an old episode of Space:1999 ("Matter of Life and Death"), where the transformation was caused by "an unknown form of radiation".
More to the point, this is also done in Anvil of Stars by Greg Bear, where alien technology allows to "reprogram" particles - alter their quantum state at a distance, changing their very nature. In the book, quarks and leptons are mostly always the same particle, their difference in charge and Higgs mass arising only by their state vector (this is a real thing in physics, you may want to look up Bohm's hypotheses). While it does take energy to flip a particle's vector state's bit by hijacking its entanglement channel (the book calls it no-ach), if you do so, you change that particle into something else - if you change a particle into a particle that existed elsewhere, that's the "classic" quantum teleportation. This solution could work for you.
(Of course, not only the general form of quantum manipulation is completely hypothetical; even the classic quantum teleportation hasn't been conclusively proven, and even if it was, it wouldn't be exactly what you need. But the description does come close).
4D printing and some satanism
This is cheating a bit, but if you have antimatter available, you could rearrange it the same way as the matter in your base object is. Antimatter seems to follow all the same physics and chemistry as vanilla matter, it's just got opposite charges.
The requisite satanism is that you will need to employ the services of Laplace's demon. You need him in order to bypass Heisenberg's principle, so as to properly place every particle. Once you've made an antimatter copy of something, you can dispose of the original if you wish.
The whole process should work to convert antimatter into vanilla matter too.
Traversable, Non-orientable Wormholes
These wormholes reverse the chirality and electromagnetic charge of objects that pass through them. Left-handed, normal-matter gloves pushed through the wormhole become right-handed, antimatter gloves. From the Wikipedia page:
if a positron can be considered as a time-reversed electron, it can also be considered as an electron aging conventionally, but with one spatial dimension reversed. The existence of a traversable nonorientable wormhole would seem to allow the conversion of matter to antimatter, and vice versa
There is no known way to do this.
If your work of fiction is following the laws of physics as we know them today then this cant happen.
But for fun let's imagine you could.
I'd propose build an "inertial dampening device", like what's used in sci fi to allow ships to manoeuvre better by scaling the ships mass by a factor suitable to the current plot. If you dial it up, your object gains mass, if you dial it down, it loses mass.
If you disable the safety and dial it down to -1, you'll have negative mass. Which with a cursory glance of physics is equivalent to antimatter. And negative mass times -1 is positive mass. Allowing you to convert back and forward. But Is the physics valid?
Nope. Its totally wrong by 2020s limit knowledge of the universe. Just state the device works in your work and move on, you're going to have to with inertial dampeners so might as well allow the technology to be used in fringe configurations. 2020 knowledge is going to be looked back on and laughed at as uninformed at some point in the future - perhaps this may be possible. Who knows? Aristotle would scoff at relativity and quantum physics, anything we write off as wrong with our puny 2020 minds might be equally right.
Make sure your interface has an "apply" button, rather than just a dial. If you dial it down from 1 to -1 and cross 0 for even the briefest of moments the object may cease to exist.