Before blowing in wall insulation, is it better to remove old rock wool insulation?
I can’t think of any good reason to keep the existing insulation in the wall. 

Blowing in insulation in walls that are already partially insulated will not produce a uniform layer of blown in insulation. In fact, there is a good chance the old rock-wool insulation has sagged in the wall cavity and will block the installation of the blown in fiberglass insulation uniformly to the stud space. 

The option to remove the wallboard will allow the removal of the original insulation in order to provide a uniform layer of insulation. In 1950 the stud depth is 3 5/8” (rather than the current 5.5” depth now) so you want it to be as good as possible to get the maximum R-value possible with the limited stud depth. 

Also, be careful of the old electrical wiring that it’s not disturbed in either option. 

However, in either case you’ll want to test the 1950’s wallboard for asbestos.