As I understand it, the grounds for objecting are exactly the same as those for an original application. In other words, it's as if the relevant change had never been made.
There's some annecdotal evidence that if anything planners are less likely to grant pemission for retention then they might have been for permission in the first place, on the basis that the applicant chose to ignore the process. Hence the advice that's usually given to apply for permission before rathar than after building.
Councils can and do insist on restoring sites to their original state, even if it means knocking down a perfectly good house to do so.