I want to create a fungal organism that feeds off of radiation from nearby stars and replaces cells within the body of other organisms with modified copies. The planet on which this fungal organism exists is near the end of its life-bearing days. The planet was once much like earth, with a molten core and highly diverse complex life. The planet's core has slowly cooled over the millenniums and as a result, the magnetic field has slowly degraded causing the ozone layer to thin and developed holes. This, consequently, has allowed more and more radiation harmful to native life to reach the planet's surface. Some life has adapted to survive the higher levels of radiation but most have gone extinct. One such surviving organism, a fungus-like species, has evolved to be Radiotrophic. This organism, which will henceforth be referred to as Fungancer, takes over animal and plant life by infecting them with spores. Once these spores gestate, they begin slowly replacing the animal's cells with duplicates that are part of the Fungancer organism. (These Fungancer cells will be functionally indistinguishable from their originals with the only difference being that they are controlled by the Fungancer) This process continues until every cell in the body is replaced, sparing only skeletal structures. Once this process has completed, the Fungancer-animal/plant will grow umbrella-like protrusions from the top and sides of the organism and move to locations where they can absorb the most radiation possible. If two Fungancer-animals/plants come into contact, they may absorb each other to create an amalgam organism.
In response to DWKraus's answer, who brought up the immune system, I would like to add that the infected animal would be suffering from radiation sickness to some degree. This would cause its immune system to be weakened.
Is such a species plausible? If so, does it have any precedent on Earth?