That is possible, but not recommended.
My experience suggests that it is nearly always a bad idea to assign a definite value to a symbolic variable after the variable has been used in an expression, except possibly when you are going to overwrite that other expression (and everythign derived from it.) There gets to be too much confusion about whether the name should represent the unresolved variable or should represent the replacement. For example,
syms x
y = x^2
x = 1
symvar(y)
will show x, and if you display x you will get 1, but y is not 1^2, it is symbolic-x squared, with there being no connection between the symbolic-x and the x you assigned numeric 1 to.
In nearly every case, it is better to use subs(), such as
syms x
y = x^2
xvals = 1
subs(y, x, xvals)
