PART ONE: To Disregard, or not?

Old Covenant and New Covenant. Old Testiment, then the New.
Before Jesus came into the world, people lived under the “Law”. Jesus died for us on the cross, and for those of us who now accept it, we live under “Grace”.
So what does that mean? Are we to disregaurd the Old Testement completely?
Would we disregard this O.T verse...”For I know the plans I have for you' says the Lord 'Plans to make you prosper, not harm you, plans to give you a hope and future” (Jer 29.11)?
Then why do we disreguard this one “ I will send the sword, famine and plague against them (His people) and I will make them like poor figs that are so bad they cannot be eaten”.?
I think we tend to read the “happy” verses as the God we know and love of the New Testement, and the “Grumpy” verses, as that mean-old bad-old God of the Old Testement... the Law.
I will tell you why I don't think this is so...