A friend of mine has an old (25 years?) Raleigh "10-speed racer" type of bike that he hasn't ridden for a long time and is thinking about getting out again. He mentioned to me that he'd like to replace the downtube shifters with bar-mounted shifters/brifters like he sees on newer bikes. I told him that it was not really feasible for a number of reasons since he'd probably have to change over his whole drivetrain, and his frame was built with narrower dropouts and a wheel size that's obsolete now.
Am I correct in my assumption? Frankly, I think he'll have enough trouble just getting replacement tires & tubes, let alone trying to find different shifters that would work.
Replacement tires and tubes aren't a problem... but it could cost as much to upgrade as it would to buy a new bike. Unless his objective is a unique bike, or he has a sentimental tie to the bike, then looking to buy a bike is probably the easier route.
I wanted unique, and I bought a 20 year old Schwinn Voyageur and had the LBS update it to modern 105 for me. Compact double 10 speed with custom built wheels, nothing too special, but wanted to stay with 27" with modern hubs due to the cantilever brakes. The build with wheels, BB, crank, derailleurs, brifter, chain, small parts and labor was $850. I used the existing brakes, headset, seat and post, a Nitto handlebar and tires I already had. I could have cut that by $200 or more by judicious buying and doing the work myself, but still not a cheap upgrade.
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
You can upgrade ANY bike if you are willing to throw enough time and money at it.
If his Raleigh it is really a true "10-speed", i.e. it has a 5-speed freewheel, it will, as you said, have narrower dropouts than currently standard (probably 120 mm instead of 130 mm), 27" wheels instead of 700c, uses a freewheel instead of a cassette and may even have Raleigh's proprietary bottom bracket threading.
If it is new enough to be a "12-speed", i.e. 126 mm dropouts and standard English bottom bracket threading the situation is better but still expensive.
Any of these things can be modernized but, by the time he changes what's needed and buys suitable newer components, he is likly to have more than the cost of a new bike in it. This is particualrly true if he has to pay a shop or mechanic to have the work done.