shifting (hah!) from downtube to integrated shifters My old racing bike has 1991-era Ultegra, but I have gradually accumulated a nearly complete set of Dura Ace 7402 (mostly new in box) and plan to upgrade the bike. 7402 was a transitional release and offered both downtube and integrated shifters; I have both. The bike is currently outfitted with downtube shifters. It is feasible to install integrated shifters on such a bike? What happens to holes and bosses for the old downtube shifters?

Generally, have people regretted making such changes to an old bike? Does anybody suggest I just stick with downtube levers?
You just remove the downtube shifters, put cable stops in their place on the bosses (e.g. Shimano SM-CS50), and then route the cables for the brifters using those cable stops like you would any other bike. Tons of people have done this and are perfectly happy with it. 

There isn't a problem with doing this so long as the shifters are matched to  drivetrain  (i.e. the cable pulls are matched, and everything is for the right number of speeds in the front/rear). 

As for whether to do this or not, completely dependent on how you feel about it. Note that you can likely make a pretty penny selling the Dura Ace 7402 stuff and running a newer Shimano Claris/Sora or Microshift shifter instead (which is likely better than the old Dura Ace shifter anyway in practice due to fresher grease, ergonomics and trickle down engineering over the past 20+ years). 

Note that Dura Ace pre-1997 used different cable pulls than the rest of the Shimano line, so you can't just bolt the Dura Ace shifters to the existing drivetrain.