Any designer needs to understand what material a product or part will be made of to ensure they can accurately design it.
Why is this important during the design phase?
I suggest you sit down with the designer in the first meeting you have and discuss what type of application the part is going to be used for. They should be able to tell you if the part will be strong enough in your chosen material. If the designer has some doubts then they can decide to do some FEA simulations to test the parts strength or simply change material grade or the manufacturing process.
Can I change material?
It is likely that you can change material but only up to a certain point.
For instance, if you first chose to design using plastic (injection molding) and then decided the part needed to be stronger so you go with metal (casting) this is possible due to the fact that the two processes are very similar.
These two process use relatively the same features and principles during manufacture so they can be changed with minor modifications.
If you decided to go with a process that does not use similar principles it could mean a redesign.
There are hundreds of manufacturing processes out there so be sure to get this right during the design phase.