Since the most common of the 1950s cymbals are Avedis Zildjian here's the story on those:
http://black.net.nz/avedis/holes.html
But then it might also be an Istanbul K Zildjian, or even an Italian cymbal. Those are the other ones I know of first hand which had smaller holes in the 1950s.
If you are going to drill the hole to 1/2" then you need to do it accurately and cleanly. I can't say how much (if any) value is lost by enlarging the hole cleanly. The reduction in value is smaller than I've been able to pick up in my research. People believe it lowers the value to them personally, perhaps in the same way that keyhole and edge dings and rivet holes do. That might be true in the general cymbal buying population, it's just that measuring the effect has proven difficult.
If you do a poor job of hole enlarging then this will likely decrease the value a bit. Again it's hard to say by how much. My initial attempts at looking at change in price by condition gave a less than clear view.
[img]http://black.net.nz/old-k/pricing/K20-price-by-cond-new.png[/img]
This is for K Zildjian Istanbul New Stamps and shows that cymbals I've coded Excellent and Good have the same price distribution, and Fair are a little bit lower. Poor (which is what you might turn yours into if you don't enlarge the hole cleanly) drops the expected median value $200 or so. Trashed is a category used for basically unplayable cymbals. But the complexity is that value changes by condition are different for New Stamps, Old Stamps, and Intermediate Stamps, and different again for other brands of cymbals.
There are some cymbals (K Zildjian Istanbul Old Stamps) which are worth more than the prices you see in that graph. There are others which are worth less. How much potential value you lose depends on what sort of cymbal it is. If you post pictures of yours I can give some guidance, or you can follow the links in my signature which will take you to my methods for identification and price estimates.
I don't particularly encourage people to enlarge holes, but I believe you deserve as much quality information as possible in making up your own mind. If these cymbals are to be used in Coffee House gigs where you don't bring your own cymbal stands then having a 7/16" hole does make life difficult. If you always bring your own cymbal cymbal stands then we can supply you with a list of the brands and models which accommodate 7/16" mounting holes, and methods to modify some brands to deal with 7/16" mounting holes.
As far as a change in sound goes I wouldn't expect any change from enlarging the mounting hole. As an experiment I enlarged the mounting hole on one of my 1950s Italian (UFiP) cymbals. It was not free to move easily on a modern cymbal stand. After enlarging the hole it moved freely on a modern stand and the sound shifted to be slightly washier and it opened up into a crash more easily. I've got plenty of cymbal stands which do take small hole cymbals so I already knew what it would sound like when it moved freely. After the enlarging of the mounting hole it sounded the same on the stands which accommodate small holes. I do a few experiments on cymbals so I can document the results.
