When you say that you update the sketch do you mean the profile itself or the sketch defining the profile's path? If you change the sketch that defines the paths, such as the dimensions in a 3D sketch, your assembly should update automatically. However, if you change the weldment profile itself (say, from an I-beam to a C-channel) you will have to redefine some mates in the assembly. If you only change the size of the profile, 1" tube to a 2" tube, the mates should update just fine.