You will need to index the new bars when you install. If you are replacing with the same 24mm, you can put them in at the same angle and the ride height will be very close.
Use an angle finder or digital level to find the free hanging angle before removing the spring plates, trailing arm disconnected. Then put the new ones in at the same free-hanging angle.
Also take time to ensure your height adjusters are mid range before you put the spring plates back on. That way you can fine-tune the heigh using the adjusters after you get it back together.
If you are installing larger tbars, they will droop less so the free-hanging angle is different. Use the following calculator to estimate the angle:
http://www.rennlight.com/cgi-bin/spring.cgi