As a reaction on my last post Besosmom2 asked me how I would change the waistband for a RTW pants that is too big.
It's a lot of trust in my knowledge (thanks for the compliment) and I would like to answer, but my knowledge on alterations is not very good. I confess to prefer making new clothes to altering existing ones.
But what I would do is try to take out the extra width in the center back, and make an extra seam in the waistband. That is how I attach a waistband anyhow, with a seam center back.
If anyone who reads this has a better solution, please mention this in the comments.