1)
Tom: I will phone you on Sunday.
Me: Tom said he would phone me on Sunday.
2)
I decided I will go to the library. (today)
I decided yesterday I would go to the library. (tomorrow)
Do I always have to change will to would when put a statement in the past? What if I say: "I decided yesterday I will go to the library." or "Tom said he will phone me on Sunday."