I think the consideration here is stem cells. The aging process involves the deterioration of DNA because the body does not produce more stem cells - as it runs out, it loses the ability to maintain the integrity of its DNA. It's tempting to think that by regularly refreshing the integrity of the DNA by uploading a configuration from when you were 20 you would stave off that deterioration, but an aging body isn't capable of following the instructions that that young DNA would give it because it lacks the regenerative cells to do so. On top of that, our genes expressing differently over the course of our lives (before senescence) is not a bad thing and you might very well cause unintended damage by trying to force, say, a 35-year-old's body to abide by the gene expressions of his 20-year-old body. What if you tell his bones to shrink (which I don't think bones are capable of doing), or what if his immune system decides that all the hormonal stuff caused by "second puberty" was a bad move? What if the sudden and regular changes in gene expression reads to his body as "we are in an environment that is so stressful that it altered our gene expressions holy FUCK PANIC!!!" and now he's allergic to everything and permanently drowning in cortisol?

I could see your idea working with the addition of new stem cells and probably a lot of case studies, but you can't treat genes like they're a coding language and the body like it's a computer. It's a silly goopy mess of hormones and microbiomes and it's a miracle the darn things work in the first place so we can't expect bodies to behave themselves however would be most convenient for CRISPR.

Final thought: if gene expressions were the end all be all of what shapes a body, you could slap some horse DNA into CRISPR and horse-ify yourself. What would actually come out of that process would be... Well, not a horse.