I have been lucky, in that I bought both of my bikes used and have been beyond thrilled with how well they both fit (and one was bought on ebay, no chance to even look at it first!). However, a big disclaimer is that neither of mine is technically a road bike so I have a little more leeway.

I had the opposite experience at a Trek factory store, where they had the bike I wanted, only every frame in the next size down was sold out. Rather than push me to buy the frame that I probably would've ended up with anyway, they suggested I call around and try to test ride the smaller frame elsewhere first. They also have a return policy if you end up with the wrong sized bike (it's either 30 or 90 days) so they STILL could've pressured me, and they didn't.

I also remember when I bought my old Cannondale, they had a company policy to let you trade-in and trade-up frames - if you had your original receipt, etc, you could send in your frame for a discount on a new, better one. I imagine you could change sizes if necessary. I don't know if they still have that policy, or if whatever company your bike is from has a similar policy