I am a light user (email, Internet, chat, photo viewing ). If I purchase the iMac with the HDD and my needs, activities change, can I upgrade at a later date?
You, as an individual, cannot. Apple won't do it either. A third party vendor may do it for a decent fee, given that pulling the machine apart isn't trivial.
Either spec it with a Fusion drive from day one, or invest in external drives/SSDs using USB 3.0/Thunderbolt to keep the speed.
I am a light user (email, Internet, chat, photo viewing ). If I purchase the iMac with the HDD and my needs, activities change, can I upgrade at a later date?
You can do whatever you want to the machine at a later date and replace the HDD. Of course it requires opening the machine in a method that isn't intended to be done by the end user according to Apple. But if you were to pop the screen off within the first year or during the duration of applecare, you would be SOL if something went wrong.