Do you believe Lightbearers are changed biologically from before they became Lightbearers?
Can't believe I forgot to answer this- I genuinely thought I did until I was scrolling through my inbox and saw it pop up again. Oops.
Anyways, my headcanon is that yes, Lightbearers are biologically different than non-Lightbearers- they become so when their ghost is rezzing them for the first time. During the first rez, their bodies are essentially restructured to better suit being a vessel of Light, with far greater phsyical resiliance, capacity, and overall durability being granted as a boon from the Traveler so that they're not torn to shreds when using even the most basal abilities. Even Lightless, they're still much stronger than a baseline human, they cannot age (their telomeres simply do not degrade and their DNA repair kit becomes more accurate, so they don't incur the gradual DNA loss/nonspecific remedies that lead to the maladies of old age), and they're still paracausally sensitive, even if their conduit for channeling the Light has been cut. Outside of the never-aging and paracasal-sensitivity thing, the effect isn't drastic, but they are still noticibly hardier than a person who'd never directly touched the light of the Traveler would be. Eris, Zavala, and Osiris could be beaten by a sufficiently strong mortal in an arm wrestling contest, for example, but they're all able to learn to use Darkness/walk off injuries with an ease that a Lightless mortal cannot.
After the first rez, additional permanant change is possible, but it requires either paracausal alteration (such as wounds created by Darkness, wounds that were earned and began healing in a no-light zone, or subconcious paracausal alteration, where the Guardian's own body rejects the touch of the Light), or deliberate ghost manipulation. I like to headcanon that by using the Light in much the same way that they would heal, Ghosts can learn to tweak parts of their guardian's bodies to be altered to their liking, which is a method by which transitioning post-rez occurs, in the instance that one was not an exo or did not medically transition before death. It's extremely delicate, but all they have to do is use their bond and the Light to go 'hey, you're missing something, you need to make more of this hormone or grow more of this tissue to fully heal', and bam, gender transed :) This fully depends on the skill of the ghost, however, so its a slow process even if it can theoretically lead to a full transition over time, and certain tissues can only atrophy so far before they need to be cut off. In the case of exos, all the ghosts need to do is block the mental disconnect between the body alterations and the perception of what those should be, and that's about it
