Thanks for your quick response, guys. Again, sorry for thread necromancy. However, this topic is indeed in my hands right now. I bought this bike with a fine pair of 19x622 rims. However, the previous owner seems to have been in a some kind of situation where he needed a Schrader / Dunlop valve to go in, so he has drilled the rim hole a bit so that a Dunlop valve will just about go in. The hole is such a nearly fit so that I will have difficulties pulling the rubber part on a Schrader valve through. A Dunlop valve works though. However, even though I don't care that much about the practical aspects of different valve types, I suffer (true story) from some OCD eccentrics. In short, it's major key that I have symmetrics in everything in life, so now my mind bleeds every time I look on the bike with a Presta valve fitted on one wheel and a Dunlop valve on the other.

This might sound ridiculousness but it's the reality I live in. I've found that there are cheap rubber grommets that you can put inside a "big" hole to make it a air tight fit for a Presta valve. However, since this is a drilled hole (probably slightly smaller than the Schrader / Dunlop hole should be according to spec.) I'm thinking that such grommet may cause more harm than good. Also, someone suggested that you could put one Presta nut inside the rim, and one outside, sort of to hold it together tightly. I asked my LBS on this and he said that the one suggesting this must be out of his mind, it was the most stupid thing he had ever heard off. He suggested the grommet solution. Furthermore, some people seem to suggest just going with it, as it is. To be honest, this is what I've done so far. I simply put a Presta tube inside the drilled rim and screwed the (one!) Presta nut pretty tight.

What's the long time expectancy on this approach, would you guys think?
I could need some confidence here!