I want to change login type of a local user to a network (AD) user
Once AD logins are working as expected I think the best answer here is to re-use your current home folder rather than trying to link accounts.

Since you're using a local home for the AD user you can shuffle the home folders and update the ownership. Be sure to do this from another account- not the local user or the AD user (you may have to create a temporary admin user).

To be clear, don't do this until you have AD logins working properly...   :)

For example:

sudo mv /Users/aduser /Users/aduser-orig
sudo mv /Users/localuser /Users/aduser
sudo chown -R aduser:staff /Users/aduser