I found that out the hard way many years ago!
For many years now I've been steaming potatoes rather than boiling them, they don't absorb as much water. A potato masher and if needed, a very quick mix with the hand mixer works well.
I thawed a bag of the frozen mashed potatoes and had them for breakfast this morning.
They were perfect, just needed a tiny bit more milk. I think the yukons were so fresh when I mashed them, they didn't need much milk. Not sure if I'd get that lucky with potatoes that have been hanging around the grocery store.
For drinking, I drink 2%, but for mashed I agree that full fat is best. I get a pint for them so there isn't a bunch of leftover milk.
Thanks again, all.