Wear it and enjoy it. The day may come that it no longer fits. You can re-sell it pre-loved. Even if you get half of what you paid, there is no real loss because you wore it and enjoyed it for all that time.
I'm late to the party, it is a lovely ring. If you hands are like your mom's or dad's mom -- ask them if their ring size changed and how. Maybe a sister to ask if she has a similar build?
Really sounds like fate! and things just working out in your favor! I don't think theres anything to worry about for now, about finding your perfect band in your size at a better price!
You all have made me feel great about this impulse buy and I've decided to keep it! If my ring size changes in the future, I'll cross that bridge when it comes to it. Thank you all for your help!