This'll never happen. One can dream, right. Hero upgrades are expensive, take forever, and we can't use them during Clan Wars or Games. They're an essential part of any attack, and vital if anyone plans on 2 or 3-starring a base during the war. Everything we upgrade in Clash of Clans is usable during Clan Wars except the King, Queen, and Warden heroes. Everyone wants this change, but don't get your hopes up.
Supercell makes tons of money from players that gem the King or Queen for war. As a result, we'll never get to use them during wars. At least in my opinion. Which is why I thought up a different approach.
What if we could use the King or Queen heroes during war or attacks while they upgrade, but with no "ability". Essentially limiting their effectiveness. No rage for the King, no cloak or healing for the Queen. The Warden won't have his special skill either. You'll still have the power behind the heroes, but not the special abilities that unlock and make them very powerful.
Some players will choose to GEM so they get the full power of their attack, while others won't. It's a win-win. I mean, you kind of have this option with the Book of Heros, but they're hard to come by.