Candi syrup, if measured by the weight of the sugar you put in and not the weight of the syrup you get out (which should be a bit higher as you're adding water), should have higher potential than LME. A one-for-one swap of the two will increase the gravity a bit, though not much if you're just doing a pound.
Does Northern Brewer send the kit grains pre-mixed or are the grains separate? If you're replacing pre-mixed amber ale grains with LME, you're changing the beer because you're losing some of your caramel malts. If your grains are separated upon arrival, replacing base malt with pale LME (probably up to about half your grainbill) should be alright. If your grains are premixed, you might want to use amber LME instead - it won't be the same as if you did the all-grain kit as-is, but amber LME will replace some of the caramel grains you're removing by reducing the grain bill.