You can migrate an OAuth1 token to an OAuth2 token by sending a POST request to the following endpoint:
https://api.soundcloud.com/oauth2/token
with the following parameters:
client_id='YOUR_CLIENT_ID'
client_secret='YOUR_CLIENT_SECRET'
grant_type='oauth1_token'
refresh_token='OAUTH1_TOKEN'
The response will contain an OAuth2 token (as well as expires_in, scope, refresh_token).