Love the Elemental series covers!
I've not been in this exact situation, but the series I'm writing just now I fully intended to be a trilogy and it's described as a trilogy on my website / book description (I think - it's been a while since I set it up). However, I've now realised that I'd like to do another trilogy, following on from this one.
Luckily it's just branded as series in terms of covers, so I'll simply make it a six book series.
In my case, it will be continuing the main characters' story, over another three book arc. So, it's right that it's just continuing the same series name.
I can see the argument for a different, or slightly adjusted, series name if there's a time gap in the stories and, as you say, the stories are going to be longer. If you want to keep the same series name, can you call the existing books novellas or a prequel series?