Nothing could be worse for a new carpet than being fitted over old underlay. It can badly affect the performance and life expectancy of your new carpet. If you already have a carpet and underlay down you will need to check the condition of your existing underlay. Is it worn or in generally poor condition?
The best way to find out is to test how cushioned your carpet feels in a busy part of the room (ie, doorways) then compare how it feels on an unused area (try under a table or sofa). If there's a big difference then it's time to change your underlay.