Yes, you can transfer legendaries.
Pokemon Let's Go Pikachu and Let's Go Eevee take a number of cues from the wildly successful Pokemon Go. The new Switch games borrow Pokemon Go's catching mechanics, for example, and in doing so introduce a major change to the Pokemon RPG formula. But on top of that, you can also connect Pokemon Go to Let's Go directly and transfer any first-generation Pokemon you've caught onto your Switch.
Before you begin, know that once you transfer a Pokemon from Go to Let's Go, you cannot undo it--that Pokemon is gone from your Go account forever. Other than that, there aren't many restrictions. You can disconnect your Go account from your Switch at any time, so if your friend wants to give you a Pokemon, they can connect their Go account to your Switch with no problem. This is a lot easier than trading the Pokemon in Go itself before transferring, as Pokemon Go trades can cost quite a bit of Stardust.
How To Connect
Connecting your Go account to your Switch is relatively straightforward. In Pokemon Go, open Settings and select Nintendo Switch > Connect to Nintendo Switch. Then, go to the Options menu in Let's Go and select Open Pokemon Go Settings. When asked if you would like to pair a Pokemon Go account, say yes. As long as both devices are actively "searching" for each other, your Switch should be able to find your Pokemon Go account and allow you to connect.
How To Transfer
To initiate the transfer, head to the Go Park complex in Fuchsia City. Talk to the man at the front desk, select Bring Pokemon, pick a Go Park, and then select Start Communication. From there, open your Pokemon list in Pokemon Go, press the Nintendo Switch icon, and then select the Pokemon you want to transfer. Remember that this is permanent, and the transfer is one-way.
Once transferred, enter the Go Park you selected during the transfer, where you'll be able to find the Pokemon. Just walk up to the Pokemon and press A to initiate catching it. And don't worry--if it happens to run away during the catch sequence, you can just try again. You also have 20 Go Parks available with space for 50 Pokemon each, and there's no limit on how long they'll stay in the parks.
How Stats Work
Before you transfer a Pokemon, get it appraised in Pokemon Go. We followed Pokemon Go Hub's guide to check our Pokemon's stats in Go before transferring them, and we found that the stats do transfer (though the CP won't be the same). Pokemon in Go have three stats--HP, Attack, and Defense--while those in Let's Go have the full six, so the stats end up mirrored. For example, a Pokemon with perfect Attack in Go will have both perfect Attack and Special Attack in Let's Go. The same goes for Defense and Special Defense. The only stat that is randomized as far as we can tell is Speed, so keep this in mind when you're catching your transferred Pokemon.
How To Get Meltan
Although the transfer from Go to Let's Go is one way, you do get a bonus in Go for doing it: a Mystery Box. This item allows you to catch Meltan, a new Pokemon that currently is only catchable in Go and can be transferred into Let's Go. You can read our full guide on how to catch Meltan and its evolved form, Melmetal, for more details.
For even more Pokemon Let's Go tips, check out our list of things you should know before starting and our guide to getting Bulbasaur, Charmander, and Squirtle in the Switch games. Finally, be sure to read our Let's Go Pikachu and Eevee review for our full thoughts on the game.