Well actually I don't, That's why I'm asking someone who knows a lot, because I'm considering buying the 8GB RAM. Thanks for the answer, and yes it seems like it would just run at the lower 2133 speed. Oh and, I have an Intel motherboard, so it's decided then.You have the means to answer your own question. Try it.
If it does work, all ram will run at the lower 2133 speed.
I put your odds of success at 90% for an intel motherboard, less for ryzen.
Would the seller accept a return if the new ram did not play nice with the old?
If you want 16gb, my usual suggestion is to buy a 2 x 8gb kit with specs like your old.
You will have a trouble free 16gb. Then try adding the old.
If it works, fine, you have 24gb.
If it does not then sell the old or keep it as a spare.