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>> a few million cells to work exactly as before but to grow more and different hair follicles as well, with the net cost of energy.
Thing is, a minimal change in the genetic code can create huge results. In some case it's just modifying a level of certain chemical that is required to reactivate dormant genes, you don't even edit the gene structure, just run a procedure that wouldn't normally run here. Not a violent modification is needed, just a gentle nudge. A tweak in proportions of proteins produced. You don't change the construction of a hair, the way it is being made, you just reduce some keratin, produce a little more lanolin, and the hair gets long and flowy and not short and curly. You absorb more water, increasing size of cells, and it gets smooth and long. You increase the amount of enzymes that trigger a place to grow hair, and it grows dense. You don't change every single cell. One in a thousand of your skin cells gets sacrificed to multiply the virus. One in a hundred gets the modified code. One in a million will have the code actually work, and initialize building the hair. Inside your body the virus will be killed by your immune system. In your skin, one in a hundred produced will actually get to "infect" another cell. The efficiency of the whole "engine" will be less than 0.01% success rate per a specimen of virus, less than one percent of your skin cells will get affected, but you will grow bushy long smooth fur...
Of course an eye in your skull is a somewhat different case. It would take years to develop (...as you say, in an adult organism such "growth of new organs" is disabled, but again, that's a matter of re-enabling it, some tricky chemistry but not impossible). Again, not entirely impossible, but again, an order of magnitude harder.