if you had purchased the BTC at $20, you now owe taxes on $980 of realized gain. Once you've paid that, your new (tax) basis in ETH is $1000, so you can do what

if you had purchased the BTC at $20, you now owe taxes on $980 of realized gain. Once you've paid that, your new (tax) basis in ETH is $1000, so you can do whatever you want with it (including exchange it to USD) if the value doesn't change.