You shouldn't "flip" the axle and use the same perches. The axle has an upward bend in it to compensate for how it will bend when the weight is on it(downward). If you flip it then you will have the arch the wrong way and the tops of the tires will start to lean in causing uneven wear.
The proper way is to install new perches on top of the axle and then bolt the axle under the springs.