Okay. If your traditional changling (as opposed to SURGE changeleing) is born human and then goblinizes around puberty in response to the mana level.
This, however, raises an interesting question. If you clone a changeling's organs in a low-mana environment, such as space would they come out as human organs?
Depending on exactly how mana causes goblinization and exactly how organs are cloned, there are potential arguments for both possibilities.
However, even if the answer is no, it should be indisputable that cloned organs created in manavoids from tissues harvested before goblinization will be human organs and will only begin to change when exposed to sufficient mana.
This, of course, allows for physical de-goblinization. Though mental attributes would remain unaltered, a changeling desiring to become human again can simply have a full-body transplant, with his brain cut and and placed into a clone body (possibly one with a cyberskull to compensate for differences in cranial structure).
Of course, there are few advantages to a procedure that simply cancels physical bonuses while keeping mental penalties, particularly since massive cosmetic surgery and dermal-deposit removal can potentially allow a troll with a good con skill to pass for a human basketball player without negating physical bonuses.
Still, for a conflicted goblinoid who wants to be human again, this seems like a the most effective way to go. Of course, he'd never be able to leave the mana void without taking the risk that his new body will goblinize. However, this can be remedied by using a generic type-O human body instead of customized cloned body.
The Type-O solution should also be available to dwarves and elves and humans who need a quick DNA change.