Make sa the owner of all databases (and add the former owner to the db_owner role)

make-sa-database-owner.sql

DECLARE @sql nvarchar(max)=N'';
SELECT @sql=@sql+N'
 
    USE '+QUOTENAME(db.[name])+N';
 
    IF (USER_ID('+QUOTENAME(sp.[name], N'''')+N') IS NULL)
        CREATE USER '+QUOTENAME(sp.[name])+N' FOR LOGIN '+QUOTENAME(sp.[name])+N';
 
    ALTER ROLE [db_owner] ADD MEMBER '+QUOTENAME(sp.[name])+N';
 
    ALTER AUTHORIZATION ON DATABASE::'+QUOTENAME(db.[name])+N' TO '+QUOTENAME(SUSER_NAME(0x01))+N';'
FROM sys.databases AS db
INNER JOIN sys.server_principals AS sp ON sp.[sid]=db.owner_sid
WHERE db.owner_sid!=0x01;
 
PRINT @sql;

---EXECUTE sys.sp_executesql @sql;