Buchanan, who co-founded the group in 1998 with original members Mutya Buena and Siobhan Donaghy, was ousted from the band last September.
The singer admitted that she has now moved on and is looking forward to releasing a solo record.
She said: "We've all moved on, you know. You have to stop looking back because you can't change things. But I haven't spoken to Heidi [Range] since.
"I'm extremely happy with where I am now. I don't know how it's going to be promoting an album on my own though, but it's nice making decisions on my own."
Buchanan's debut album is expected later this year.