We bought two separate apartment buildings ( for a total of 4 buildings and 35 units ) all with existing tenants and it has been going very well for us. We only have one problem one so far. Not bad for out of 35.
The one apartment building has some very low rents. We honored their leases, then when they were due for renewal we raised the ones who had not had an increase within the past two years, and left the ones alone who just had an increase last year. We haven't had anyone leave yet and everyone seems happy that we didn't come in and 'throw' everyone out. We hosted a meet and greet when we took over, and nearly every tenant was more concerned about still being able to live there as opposed to any other issues. They didn't want to have to leave. Maybe we were just lucky. LOL
I still have one of the older tenants hug me every time she seems me because she is so happy she can continue living there. Her place is immaculate and is never late.