I read in a forum a few years ago where this guy came up with an ingenious way to acclimate his betta s to backgrounds.
He took a few pieces of paper tha same color as his wall that the tank was against, and taped them together, and cut them the same size as the back of his tank.
Then he cut a small square hole about 2 inches square in the cente, placed his background behind it, and attached to his tank.
Every few days he cut the hole a little larger, until, about a month later, it was entirely all new background and the fish never had a problem.
But you know bettas, what works for one may not work for another.