Personally, I think if people are willing to pay for a username change and the name happens to be taken by an account that hasn't been active in x amount of years, then that account should be pruned. I believe it's only fair since they are actually going to be paying for the name.
Ultimately, I think this would be up to the admins to decide on.
That said, I actually think a prune in accounts should happen regardless. I know it doesn't affect the server on how many unused accounts we have, but for the sake of a more clean and organized forum, I would rather see accounts that have at least 0 posts and have not been active in perhaps 3 or more years be removed. But, this is another matter.