You need to know the axle to crown measurement of the fork coming off the bike, compared to the same on the new fork. Chances are, it will be greater on the replacement fatbike fork, which puts the headtube of the bike higher off the ground than stock.

Add the fat tire to it and you further increase the distance of the headtube from the ground

That slackens the angle of the headtube leading to slower steering. A function of that slack head angle is also an enormous increase in trail, which makes things stable, but also slower turning.

If you want to get a taste of what you'd be getting into, find a 26" wheel that fits your frame, swap things over, and see for yourself what happens to handling.