I suggest reading the chapters in "Adventures in Tandem Nursing" by Hilary Flower, "Mothering Your Nursing Toddler" by Norma Jane Bumgarner and "The Womanly Art of Breastfeeding" by LLLI that cover nursing through pregnancy.
I agree with the pp who said that it's different for everyone...I'd like to add that it's different for every pregnancy, too!
My ds was 9 months old when I got pg. At the time I got pg I had oversupply issues, which were resolved by the pregnancy! I had plenty of milk during pregnancy and tandem nursed following ds#2's birth. I didn't experience any discomfort at all while nursing through pregnancy.
Ds #2 was 15 months old when I got pg with dd. I tandem nursed my 2 ds's through dd's pregnancy. That time around I had sore nipples for several weeks, and sore breasts for a couple months (whenever someone touched my breasts, it hurt....wierd!). I didn't experience a drop in supply. I triandem nursed for over 18 months after dd was born, until ds#1 self-weaned.
I'm currently tandem nursing ds#2 and dd and am pregnant and due in September. I'm about 8 weeks along and both dd and ds say that there's still plenty of milk. I don't have sore nipples this time around and things are going smoothly.
During pregnancy your milk may change in taste, but you need to remember that your milk's taste is always changing, depending on what you eat. Toward the end of pregnancy, colostrum comes in, and some older nurslings don't like the taste. My kids loved it, and called it "mamajuice" instead of "mamamilk"! It gives the older nursling(s) a boost in immunity and all the other wonderful components of colostrum, and there is plenty for baby as well!