You can already replace the battery. You can purchase a replacement here, along with all the tools you need to replace it if you don’t already have them: https://www.ifixit.com/products/steam-deck-battery

There’s also a full video guide on the process.

Valve worked with iFixit to make sure they have replacements available for every part in the deck. Valve was very up front that they wanted to enable people to do their own repairs and upgrades.