I understand that, if you were an elder still, you would formally withdraw from being a trustee (ie the congregation secretary would not just delete you, but a letter would be written to the Charity Commission.)
And presumably if you were to be made an elder (ie what would have been a new trustee), a letter would to sent to the CC confirming you were not a trustee.
https://drive.google.com/file/d/0BwqmWMK7dwtlSG40YUg2RTJPZVE/view
https://drive.google.com/file/d/0BwqmWMK7dwtlMWxVaFg5V01uWFU/view
Good info - thanks DS