It’s a gimmick and like most gimmicks I’m certain it’s going to fall flat.I am completely aware this won't change the show, I don't think at all this is in any way a way for them to change the show but merely a way to get people to watch as I mentioned the curiosity factor playing a role in. My entire post is about why this was an optimal decision in terms of ratings. Whether or not this is also intended, it will in a way "condition" viewers to possibly ease into the format as Masterchef was able to do with the new judges. Having All Star Masterchef contestants were able to hold onto the initial Masterchef feel despite the judges not being there, through association audiences were able to ease into having new judges. As with this some viewers who have turned off may find they will be inclined to enjoy the show due to simply seeing familiar faces they enjoyed seeing in the past. Classical conditioning is fascinating thing that can be used in a showed in more ways than just editing. Some viewers my genuinely tune in to see former housemates and some of those will definitely stick around to watch the entire season. This ratings gimmick may not be what I would prefer but I have no doubt in my mind it will work for viewing interest. But also you can't not expect some form of content to make the show showing the past housemates discussing their 20 year old selves the first time in the house compared to now.
One of the major criticisms of this format is that it has lacked a clear identity and is stuck between being a game of social strategy and the BB of old. Brining back alumni is production yet again doubling down on their failures.
As for ratings I can’t see it doing much at this point… The novelty will wear off quite fast.