Once multipart features participate in a geometric network, their geometries can only be modified to create valid network features. Since M and Z values are vertex attributes of geometry, they cannot be modified.
To modify M or Z values, one of the following two solutions must be used:
1. Drop the network, modify the M or Z values and rebuild the network.
2. Create a valid network feature.
To create a valid network feature from the multipart feature in versions of ArcGIS 8.3 and earlier, see Geodatabases > Editing your geodatabase > Repairing network topology in the ArcGIS Desktop Help.
To create a valid network feature from the multipart feature in versions of ArcGIS 9.0 and later, see Editing in ArcMap > Editing geometric networks> Repairing network connectivity in the ArcGIS Desktop Help.