when you delete a member you will be prompted with a couple of options.
Warning: Deleting a member can not be undone.
Do not delete this member.
Delete member and do not alter posts. Ownership will be set to Anonymous with an Anonymous name of: sb.
Delete member and delete all posts. If there are any replies to a post by this member, they will be attached to the post's parent. If the user's post is a topic, the entire topic will be deleted.
I have not tried to preserve a post in quite some time because I only delete members with 0 posts.
But I do recall that pre 7.5.6 did have a rare bug that option two would not keep the name but change it to Anonymous
So you may want to test it on just one member first
