Can the charge of particles spontaneously flip from positive to negative or vice versa? I'm thinking of matter antimatter annihilation, are there reactions where normal matter converts to antimatter?  
 A: Short, short answer: no.
Short answer: That would violate various conservation laws.
Discussion:
To begin with I want to exclude from discussion those particles that are their own anti-particles--the force carrying bosons and the compound particles like the neutral pion that are symmetric under the charge-exchange operator $\mathrm{C}$--or those like the neutral kaon's that David discusses which are connected by weak rotations in flavor space.  I'll discuss one exception to this exclusion below
The rest of the charged fundamental particles (i.e. quarks and leptons) all carry conserved quantum numbers that are different for the positively and negatively charged versions.


*

*Each quark and charged lepton carries charge (yeah, duh) which is conserved by the electromagnetic interaction.

*Each quark (down, up, strange, charm, bottom, top) carries at least a color (red, green, or blue for the matter quarks and anti-red, anti-green, or anti-blue fr the anti-quarks) which is conserved in strong interaction.

*Each charged lepton (electron, muon, tau) carries the (imaginatively named) lepton number (+1 for the negative leptons, -1 for the positive leptons) which is conserved in the weak interaction.
But what about the neutrinos?
That's the rub. As thing stand they may or may not be Majorana particles and therefore may or may not change their matter-ness when they change helicity. The primary method in use to try to sort out the Majorana/Dirac nature of the neutrinos (neutrinoless double beta decay) is only possible if a neutrino effectively flips it's matter-ness. An answer should be forthcoming soon as saerches are on.
A: There are reactions where normal matter converts to antimatter. For example, in neutral kaon oscillations, a beam of kaons (or rather, what are created as kaons) will appear to convert to antikaons after some distance, then back to kaons, and so on.
However, a key property that allows this to happen is that the kaons are neutral. This wouldn't happen with charged particles, because electric charge is conserved. The flavor identity that distinguishes (neutral) kaons from antikaons, on the other hand, is not conserved.
So no, the charge of a particle cannot spontaneously flip from positive to negative or vice versa. But it is possible for matter to turn into antimatter.
