... My question is, that receiving address that I gave out two days ago is still unused, and even though the current one showing is different, the unused address I gave out the other day is still ok to receive the payment to, correct? ...
Assuming that the "unused address" you gave out the other day is actually contained within your wallet, then yes... it should be OK.
You should either:
1. Look at the address tab (You may need to goto "Wallet -> Address" to get it to show)... is the address you gave out listed there?
or
2. Goto the console... and type the following command:
ismine("PUT_THE_ADDRESS_YOU_GAVE_OUT_HERE")
If it says "true" then yes, the address is yours, it'll be fine... if it says "false" then DON'T get coins sent there until you figure out where that address is from!