You should create grid question instead of free format after showing the product concept. This grid should be column-wise format, first column (your old ranking (1-5) )should be numeric response only and optional, second column (your new ranking (1-5) ) should be as ranking type. Now you should put some java-script code for first column, where you need to show result from previous question and disable these numeric box. So User only can see their previous rank cant modify.
Let me know if you need code for this.