Personally, if you like the existing bike (frame geometry) which it sounds like you do, I would keep it and upgrade some parts.
For instance, I would definitely get lower gears. Maybe some nice road tires (Schwalbe Supreme or similar) since the TA is 99% paved. If the current wheels have held up to the same weight or more as what you will tour on, I probably would not change that either. If the brake and gear cables have never been changed, perhaps switch them out to new cables and housing because 15k on a bike is pretty unusual and the cables do wear out and will eventually snap (Murphy says it will be in the area where the closest bike shop is 200 miles away and it will happen at 3pm on a 3-day holiday weekend in a small town where the post office is only open on Monday, Wednesday, and Friday). Same with brake pads if you use rim brakes. Of course, you should learn how to replace them yourself can at least carry a spare set of cables so that if it does happen on the road, you can keep riding. Also, are the racks strong racks or a cheap flimsy one. If the latter, I would get decent new (or decent used) racks. Used decent racks should still have years of life on them unless the previous owner used them to carry 60+ pounds continually over gravel roads. All of this is way less than the $2k+ you will spend on a new, untested, bike.
Anyway, after 15k miles and several 2+ week long tours, you would know you like the bike. Why risk changing it for something you may not like. If you do buy a new bike, I would at least keep the current bike until you are sure you enjoy the new bike.
To me, it sounds like the LBS is just trying to make a sale.
Wishing you a wonderful time on the TA. Tailwinds, John