So, about auto message...

[IftiBashir]

Why can't you just upload a video explaining you've made a new channel on the old one?

Very good idea - it would reach out to your subscribers, and the active ones at that, and you give people the choice to check it out, rather ten forcing anything upon them. PMs will be regarded as spam and may well put them off of your channel altogether!

 

[Aural Engineer]

If your old channel has the intro as "go to my new channel" video. There's no way I can see it not doing what you want.

The thing is that some people subscribe to your channels without ever checking your channel page. You can notice that behaviour espacially for music channels.
So this message is for all the ones who only wait for new content to pop up at their welcome page.
Anyway, it doesn't really matter to talk any further because I don't think anyone can help me here.
Thanks for the effort though.