Side Note :Obviously backup what ever is on there!
Back on track: Yes, you can replace the first disk with a bigger one, then the second at a later date and expand your LVM to fill out the remaining space. Read up on how to do this, depending on your set up; read about LVM growing/expanding, fs expanding, also how your hardware RAID card work in this set up (it might not allow the growth of existing volumes?). Its perfectly achievable just need to know how for your specific situation.