Understanding and updating beliefs on deeply engrained topics can take enormous efforts, but sometimes it can be so hard that the listener cannot even in principle accept the new reality. The listener is simply not ready, he lacks a vast background of reasoning leading to the new understanding.
What fact that you know is true but most people aren't ready to accept it?
By "you know is true" I really mean "you are very confident to be true".
Feel free to use a dummy account.
Woah, woah, slow down. You're talking about the edge cases but have skipped the simple stuff. It sounds like you think it's obvious, or that we're likely to be on the same page, or that it should be inferrable from what you've said? But it's not, so please say it.
Why is growing up so important?
Reading between the lines, are you saying that the only reason that it's bad for a human baby to be in pain is that it will eventually grow into a sapient adult? If so: (i) most people, including myself, both disagree and find that view morally reprehensible, (ii) the word "sapient" doesn't have a clear or agreed upon meaning, so plenty of people would say that babies are sentient; if you mean to capture something by the word "sapient" you'll have to be more specific. If that's not what you're saying, then I don't know why you're talking about uploading animals instead of talking about how they are right now.
As a more general question, have you ever had a pet?