This is still a little confusing to me (partly because I think I never actually saw the RT protection factor kick in), but here's what I get out of it as it applies to most consumers who buy tickets well ahead of time at the lowest cost possible:
Changes to existing tickets will now be equivalent to refunding the price of the leg in question to your My Wallet, and then immediately buying the new leg at the lowest available price. If there's money left over, you keep it in your wallet, if there's money due, you pay for it immediately. It doesn't matter if you're changing the outbound or inbound portions, or whether you've started your trip or not. Is that about right?
It makes the rules consistent but means that extending a stay by a few days after your trip has started will now likely be much more expensive than before.