@BHStowel, if the Camden has gone through the approved process of advertising its proposed change in admissions criteria to prioritise Camden pupils, then published the new criteria in accordance with approved timescales, that is absolutely fine. Islington applicants may feel upset, but they have followed due process.
However, if @flippyfloppy has applied to a school that has no published priority for applicants in the neighbouring LA, and has applied according to the oversubscription criteria, with the expectation that her application would be dealt with fairly according to that criteria, then she has been placed at a disadvantage if it has been downgraded because it is 'out of borough'.
Therefore, she should appeal, on the basis that her application has not been dealt with according to the admissions criteria, which has cost her child a place.
@flippyfloppy you must telephone the LA on Monday to alert them to their mistake and ask them to rectify it. Ask the person you speak to for their name and their email address. Follow this up with an email, outlining your conversation and confirming that you are requesting that they rectify their mistake without you having to take it to appeal, but that you will appeal if they are unwilling to do so.