IMHO you're not going to lose too much value by shooting it. However, if you put a significant amount of rounds through it - think thousands - then yes, the value will be negatively impacted.
HK's hold value well. Enjoy it, but don't abuse it. Remember that we rate guns based on percentage of condition, with 100% being new. Once you fire the gun the percentage rating will drop, but not much...unless you tear it up. Most of us who collect can rate a gun on the spot.
You are in control of the value. Take good care of it and the value will remain high. You paid $675. I cannot tell you what the resale will be until I actually see the gun and assign a percentage. That's exactly what other buyers will do, but it's nothing to stress over.
Just my $.02. YMMV
Have fun!
Take care,
DFW1911