Mainline Vent openings too Small?

I have two main vents, which you'd think would be adequate for my smallish system, but they are only 1/8" each (radiator size). Is it advisable that I tap these old pipes to accommodate 3/4"? My system seems to be working fine now, but this might save me some dough in the long run. What are the risks in tapping an old pipe like this? Basically I don't want to create a disruption/leak in the adjacent pipe joints, etc. Thank you.