According to The Verge, this DIY kit will let you use your old Nintendo 64 controllers to play on your Switch! 8BitDoβs Mod Kit helps you to replace the circuit board in the original controller with a Bluetooth-compatible version, so no you can reuse your original hardware without having to track down an official wireless gamepad! BitDo is selling its DIY kit in a couple of different variants starting at $29.99. 8BitDo says,
the kit can be installed without the need for any soldering, includes an LED status indicator, and should offer 8 hours of playtime on a single charge. You charge it via a USB-C port on the included rumble pack, and because the mod involves removing the original controller cable you get a small plug for the hole.