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In broad terms, I'm wondering if it is possible to repair a high quality lugged carbon frame, say a Colnago C60, that has been cracked in the top and/or down-tube.
Normally once a carbon frame has a crack or chip it is considered unsafe to ride and it will usually get scrapped. However, it seems like if one were to crack a lugged frame, it might be possible to take it apart and replace only the tube that has cracked.
Is this possible and if so what does the overall process look like?
It should be possible to repair the frame, the big question is whether you'll be able to find a replacement tube or will you have to make it yourself. Making the tube isn't hard, it's just messy.
The repair will involve cutting out the damaged tube, working out what else (if anything) is damaged, and paying careful attention to the other tubes. Then once you're clear on exactly what needs to be done (and that the person paying for it is happy with the price) you start. This may end up cutting an undamaged tube so that the new tubes can be fitted, depending on just how rigid the frame is (it should not be that rigid unless you have a very short tube)
I would be tempted to use a slightly extra thick tube to get a little more strength, since you broke the last one. But that's a path that can lead you to completely rebuilding the frame.
Colnago's EPS and C59 are both lugged carbon fibre bike frames dating from the 90s, when CF was a new material. They had a reputation for unreliability (I don't know if that was the construction, or something else.)
Whether you'll find one new, or used and in good condition is a completely different problem. The lugs and the tubes are fastened together with epoxy, which makes swapping out a tube difficult.
Also, the stress point on the CF tube is now where it enters the lug. The modern monocoque construction of a CF frame does away with those stress focus points, so "should" last longer. Plus popping the frames out of a mold in one piece is a lot less work than assembling and epoxying tubes and lugs.
Answer: No you can't just change tubes, because the tubes are epoxied into the lugs, and you couldn't change just one because the angles of the lugs would require all joints loosened to slide apart.
Homework: OP should consult with a local carbon frame maker/repairer who can give a definitive answer.
