what do you mean by "the positions don't change, [...] make the symbols change"? So you have a plot, which does not change, but you want the colors/markers to change?
Generally speaking, you can either simply replot your data points using the new desired colors/markers, or you can use get/set to change any values/colors/markers etc. you like. The general formulation would be something like
handle = plot(x,y);
% then change whatever you like
set(handle,'option_name',value)
you can get a list of the available options by typing "get(handle)" (where handle is your actual plot handle).
