Disagree with the simple answer from the 'head of property litigation'
Code powers do not give Openreach carte blanche, there will have been a way leave in place for the previous poles, and going underground ducted or direct buried is not likely to be covered by that way leave agreement.
What will usually happen is an enquiry to change and if all parties happy then wayleave updated. Falls apart when private land owner asks for compensation and both parties disagree on the amount, at which point the eventual resolution is via a court (this is a lengthy process and is being simplified to reduce costs i.e. lawyers fees)
Getting compensation is normal. Mobile masts on a roof used to be a good income stream for building owners, but believe an agreement has been reached to cut this back to a flat rate of £5,000 recently.