Depends on how far you want to take this, I am only aware of 1 person making a legal challenge (so they say) because the engineer wouldnt release the engineers code/ or reset it to the default.
Based on that case I would say it is possible to make the engineer change the engineers code and provide it to the end user.
However I would expect a fee to be paid, that fee should be inline with there fees to attend site and perform a minimum duty, as the process is short it should not cost any more than there minimum. Any system where the engineers code is disclosed to the end user loses their right to any warranty that would normally be offered.
In my opinion it would be unreasonable for an engineer to disclose an engineers code that compromises all his systems, or for any system they are offering any guarantee or responsible for. All my clients that request the engineers code are told that by doing so there is no warranty as I can not guarantee the integrity of the system it then becomes a choice. The engineers code where its displayed in full prior to being changed, is changed before the end user changes it to their preferred code.
That is reflected on their receipt.
I went and fixed a system last week changed the engineers code, the customer said and I quote "I want the engineers code so I can maintain the system".
Which you may consider a bit strange because they were playing around with the system, got it in a mess and called me to sort it out in the first place.
Fine, I changed the engineers code to the default and let them change it to something else (they let the property), showed the system working and advised them that beyond that I can accept no liability for any future issue as I have no control of the system.
The codes can be defaulted on that system even if its a ball ache by a competent installer.
My customer is always right, but as a result of their decision may lose some of their rights to warranty.
You simply cant expect an engineer to be responsible for a system if the end user has access to the engineers code.