Upgrade from say a 7-speed to a 10-speed (because you have 3 discs in front) without changing anything is a double question :
1. Does your rear derailleur is compatible ? Meaning : is the cage length long enough and does it have sufficient amplitude.
2. 10-speed chainings are thiner than 7-speed in order to avoid friction on the front discs.
You have an example with the Shimano 640x group (great stuff btw), the rear derailleur is the quasi exact same design but when the 6400 one is a 6-speed, the 6401 can handle 7 to 8 speeds just as easily provided you mount a thiner chain.
This is a technical spec problem from the time, if the 10-speed chaining was very rare (or inexistent) at the time, you would have to calculate derailleur amplitude from a 6-speed point of vue. So now that 10-speed chainings are available, if your discs are thin enough (usually they are) you can pass the 8 speeds on the bike just by changing the chain.
It's theoretical from a MTB point of vue but it's a Roadbike reality.
The morale is here that it's not because it's old that it's obsolete, sometimes au contraire.
I upgraded a 70's roadbike with a 90's group, my main problem was that some setting screws were not designed for ulterior frame modifications which impose a workaround (basically a longer screw and adaptation of the stop).
The thing is, what is your desired upgrade ?
1. If it's "the same thing but younger" it should be fairly easy because things have not changed dramatically between the 90's and today (less than between the 70's and the 90's)
2. If it's "more performance at all costs" it could be tricky because it means changing at least the chain, cassette and shifters but also eventually the rear-derailleur and front discs (if they are too thick you will have some nasty friction).
Concerning the shifters, unless you have good old fashioned friction ones (this kind : https://images.jet.com/md5/f636d8a5af21fce219fa5fb3809f6198.1500) it could be tricky too because if it's an indexed system, provided that you stay in the brand and that you keep the same number of speeds you can certainly be lucky (because it's more of a cassette issue in reason of the system, indexed shifters provide differential tension which is calculated from the inter-speed space which is proprietary).
You don't have this problem with friction shifters because it's an analogic device that does not care at all how many speeds you have (because if you tend the cable all the way you will span all your cassette anyway). Same thing for the front.
This is why (I'm sort of lazy this way) I systematically use friction shifters, it's bit tricky to use at first when not accustomed but you get the coup de main pretty fast.
But if you have now a 7-speed indexed shimano transmission, upgrading the shifters with newer 7-speed ones should not pose problems.
