To be honest, I don't think they will interfere! Artists get better with time, and in webcomics it's even seen as a plus by many to be able to go back and see how much the artist has improved. If your other series are completed, by all means, leave them up, because a finished series is always a plus point. And even if they're not - I have a hard time believing that readers would unsubscribe from a comic they like (your new work) because they're not that into your older work.
Oh and another thing: Differences in art skill are always the most obvious to the artists themselves - most readers don't think as much about it. The artist is digging into the details of the drawing while making it, while the readers just, well, read.