HI Aaron,
Since Master data is created in a hierarchical manner, you cannot change the business entity of buildings.
Fetch the data of buildings with BAPI : BAPI_RE_BU_GET_DETAIL and copy it in any file.
Or you can also take a dump from database table of buildings.
Once you are ready with the data use BAPI : BAPI_RE_BU_CREATE with either SECATT or LSMW. Else you can also use this BAPI with the help of ABAP to upload the data with a new business entity.
Hope it could help you!
Regards,
Siddharth Khandelwal