We're told (in *[Buffy: Real Me][1]*) that a vampire can only enter a property when it's invited in by a someone that lives there.

> **XANDER:** *Uh, yeah, actually, she -- Harmony -- kind of happened to sort of get an invite.* 
> 
> **BUFFY:** *You guys can't invite her in. I mean, **only someone who lives here can**-*
> 
> Buffy: Real Me

The barrier on the property ceases to function [if and when the human residents die][2] and, critically to your question, **a new barrier is erected if new human occupants takes up residence in a property that was previously accessible to the vampire**, as seen in *[Angel: Heartthrob][3]*, where Angel, who was previously able to do a room-by-room check of the Hyperion, is now suddenly barred from entering Fred's room.

> **Angel:** *I can't come in.*
> 
> **Fred:** *Of course not. You're worn from your trip. You go rest. We'll catch up later.* 
> 
> **Angel:** *Fred, I *want* to talk to you, I just can't come in unless you...*
> 
> **Fred:** *I invite you. Instead of being rude! Oh, come in. Come in.*

Note also that mere *ownership or technical residence* of the property isn't sufficient to bar a vampire from entry, as seen in *[Buffy: The Freshman][4]* where vamps enter Buffy's dorm room freely, as contrasted with *[Buffy: The Yoko Factor][5]* where, a few months later, Angel needs an explicit invitation to enter the same room now that she's accepted it as where she lives.

  [1]: https://buffy.fandom.com/wiki/Real_Me
  [2]: https://static.wikia.nocookie.net/buffy/images/f/f9/AngelLeansOnInvisibleWall.gif
  [3]: https://buffy.fandom.com/wiki/Heartthrob
  [4]: https://buffy.fandom.com/wiki/The_Freshman
  [5]: https://buffy.fandom.com/wiki/Yoko_Factor