I would have asked them if they could have gotten the chosen bike for you explaining that was the only one you wanted. If the answer, as expected, was no, then you would have been off the hook. If yes, and the price was right, you could have stayed with them. Either way, you wouldn't have been in the current predicament.
As it is, I don't think you have a problem. One approach is to be bold and take the new bike into the old shop to show it off like you would do with any "friend". That would be assertive. You would be telling them they are important to you, you just couldn't do this deal with them. Sweeten the visit by buying something significant while you are there. I think they will understand you value the relationship. In my experience shops are very gracious under such circumstances. This would be better than sneaking around hoping they won't see the new bike.