There are no problems with welding investment cast lugs, as these are steel and present little difficulty. Only issue is the heat of the welding process is a great deal more then that of the melt temperature of the material used to make the frame, and even though all seems perfectly ok it is possible the brazed joints will have been weakened by the welding carried out.
If you are sleeving something like an investment cast headstock, to take different bearings, then TIG brazing is the best process to use, as this puts the least amount of heat into the job, and greatly reduces the chances of any problems with the original brazed joints.
An example of what can happen when welding is carried out adjacent to lug brazed joints, was a BSA MX frame which had had welding carried out near to the front motor mount, and which unfortunately resulted in the brazed joint failing after landing from a big jump (there was a picture of this on the net several years ago, but not sure its its still to be found).