For this to work, SoundCloud will need to know about the updated redirect_uri. I expect that this can be done at https://soundcloud.com/you/apps, but I can’t check because SoundCloud aren’t currently accepting new developer signups.
The redirect link for an existing application cannot be changed, and as you mentioned no new application can be registered. So I'll have to leave it a HTTP.
For this to work, SoundCloud will need to know about the updated redirect_uri. I expect that this can be done at https://soundcloud.com/you/apps, but I can’t check because SoundCloud aren’t currently accepting new developer signups.