DivinylsFan Saving the household to the library and placing it in a new save is perfectly fine. What is bad is not quitting to desktop before loading the new save. If you only exit to the Main Menu, some of the data from the old save can remain in memory and be superimposed on the new save, corrupting it before you've even started.
This is also why using the do you want to move to a new town pop-up is discouraged, and the same goes for using the phone or computer to move to a new town. You're doing the same thing, loading a new world without closing the game, and thereby running the risk of corrupting the new save.
And to be clear, the old world doesn't need to be corrupt for its data to corrupt the new save. The simple fact that it's a different world (or at least a progressed copy of the same world) means there will be mismatches in pointers and other details that are impossible for the player to see but that the game depends on in the background.
Point is, save your household to the bin, quit the game, load the new save, and place the household there. If you want to preserve their house, save it separately, which you can do by evicting the household first.