Typically in a Legend of Zelda game once you pick up a heart container, that's it - that heart remains with you forever, and that's that. Link is upgraded and it stays with you to the bitter end. This doesn't have to be so in Breath of the Wild - just another way in which the game subverts series expectations after a great many games using the same base formula.
Heart Containers and Stamina Upgrades are earned in Breath of the Wild through completing shrines, with every four spirit orbs you pick up able to be traded at statues of the Goddess Hylia throughout Hyrule. At these statutes you get a choice: heart container or stamina vessel?
But here's a rough question: what if you make a mistake? What if, as I did, you discover the Master Sword requires 13 hearts to retrieve when you've already spent 12 spirit orbs on stamina upgrades? Well, hidden away in Hyrule... there's a method to change all that. But it is indeed well hidden.
How to Swap Hearts for Stamina and Stamina for Hearts
First off, head to Hateno Village - the nearest shrine to warp to is the Myahm Agana Shrine, which basically sits right next to the village. Zoom your map right in and have a look around this area: right near the shrine and to the south of the Hateno General Store is the abandoned house that Link can buy if you're so inclined.
Between this house and the town proper is a bridge over a pond - Firly Pond. This is where we need to be, and you can see a specific image in the screenshot we've posted just below.
The Statue's Bargain Sidequest
At this position you'll find a shrine statue, as pictued above in this article... and you should be pretty used to those right now. But waht's this...? This one has horns?
Chat to it and it'll kick off a sidequest - The Statue's Bargain. Here's what'll happen: The statue will steal one of your hearts, gloat a bit about how evil it is, and then... that's that. Chat to the statue again and it'll offer to give you your 'essence' back, though this time you can now choose which you want back: a heart container or a stamina vessel.
This marks the side quest as complete, but from now on you can pay this statue a visit to swap your health and stamina round - making it easy for you to max out your stamina or max out your hearts through swapping if you're so inclined, or making it easy to make the game a little harder.
Going forwards, the statue will pay you 100 rupees for each sacrifice you make, but then when you want a heart container or stamina vessel back you'll have to pay 120 rupees, evening it out to 20 rupees each time you want to swap the two over - not bad, all told!