There are good reasons for objecting to houses being turned into flats which have nothing at all to do with snobbery.
Planning permission aside.... if you're in a terraced house and nextdoor's conversion means that what was the front bedroom becomes the living room, you're likely to get extra noise. If you get a big telly in there and regular house guests up late, they don't have to be that antisocial to cause you a lot of noise and disruption. of course, you can have a huge telly and lots of people up late in a bedroom - but it's less likely, unless you are talking bedsits.
Check on the plans where new staircases and doors will be installed. This will mean footfall and doors slamming etc where you didn't previously have them.
Parking can be an issue if the property has no offroad parking, or even if it does. You don't need to be a genius to work out that having 2 or 3 cars per household means a lot more competition for space than 1 or 2 per house.
It's also worth thinking about what is actually needed in terms of local housing. Round here, there is a lack of affordable family homes, but a lot of one and 2 bed flats, for rent, not sale. There's been a big rise in buy to let mortgages and conversions along the lines the OP describes. It is all about making money for landlords, and has nothing to do with a pool of affordable housing. You don't have to be a snob to object to this.