The new landlord is going to have a lot more invested in the property than the old one so, to achieve a market rate of return, they most certainly will have to raise any "grandfathered" rents. And, if they invest further in upgrading or updating the building, they will expect increased rents to provide a market ROI on that too. Otherwise, there is no incentive to do so.
Some landlords don't want to go through the hassle of pouring more dollars into a property either because of the effort, lack of funds or uncertainty of an appropriate ROI. In return, the tenants live with a lower quality space and/or participate in the maintenance of the building. This is a mixed blessing for the landlord as tenants are rarely going to maintain a space like the owner would. Often, this leads to a continuing downward spiral for the property. At some point, the landlord wants off this treadmill and sells to someone who is more willing to step up as a fully professional landlord. But, again, this comes with increased rents, often driving out existing tenants. For the new landlord, they would spell this out as "opportunity."
Tenants often forget the maxim, the rent pays for the property. If it is too low, there is a day of reckoning.