You can replace the ABS module without disconnecting the brake lines, I just did mine because the ABS light was coming on intermittently. It is a little awkward to do though. To do it you have to remove the bracket that supports the HCU. First remove the bolts that hold the bracket to the car, 2 on top and one underneath. Then remove the bolts that hold the HCU onto the bracket, this part is a little difficult because the one closest to the radiator has very little clearance and needs to be removed from underneath. Then you can remove the bracket and let the HCU "hang" by the brake lines. Now you can fit a torx bit on a 1/4" socket wrench and undo the 6 screws that hold the ABS computer module onto the HCU. Put the new one on and reassemble in reverse.
Hope this helps...