Substitute decision variable with a constant value

[robingizmo]

Is it possible to fix a variable (x) to a constant value (x=1) in an LMI that was previously defined, such that the original LMI does not need to respecified. I.e. is het possible to change the entries of a previously defined LMI and specifically the decision variables, or should the LMI be explicitly restated?

[Johan Löfberg]

replace(myexpression,x,1)