Do not try to add ram, plan on replacing with a single ram kit.
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, can be very sensitive to this.
This is more difficult when more sticks are involved.
That is why ram vendors will NOT support ram that is not bought in one kit.
If you do buy more disparate sticks, they must be the same speed, voltage and cas numbers.
Even then your chances of working are less than 100%
What is your plan "B" if the new stick/s do not work?
Sometimes increasing the ram voltage in the bios will make things work.
If you want 32gb, my suggestion is to buy a 2 x 16gb kit that matches your current specs.
Then, try adding in your old 16gb,
If it works, good; you now have 48gb.
If not, sell the old ram or keep it as a spare.