To get the old flock off there, I use two ways. Firstly, I just scrape it off with an exato knife until most of it is gone. This seems to work very well, but I do get a couple of knicks on my fingers. After that I just use a thinned glue and paint mixture and flock back over the stuff.
Second, when I want to get a "nice" effect. I do the same thing, but use Squadron Green Putty to build up the base a little. It is very thin and you can smear it on with a sculpting tool. Don't let it hit paint, it will disolve it.
All of my new snow bases have mounds of snow, built up with Squadron putty. I just sand it smooth, paint it with a thinned blue green paint for the first coat. The second coat is white glue, white paint, and dip the model in snow flock.