Why not start by replacing just one of your meals with Whey and then if that's ok, go up to two, then three and so on. That way you give your body time to adjust, and hence you don't lose a lot of the benefits by getting the runs.
You should not have any problems unless you are Lactose(milk sugar) Intollerant.
If you can drink lots of milk without gas, stomach upset you will be fine.
Whey concentrate has about 10-15% Lactose.
Whey Isolate has < 5%