Walker, 26, has spent the past six years with the Rams, making his debut in 2007, having signed from Shaw Cross Sharks.
He will now join up with the Eagles, who are one game away from a second successive Championship Grand Final appearance.
Winger Buchanan, who is the Rams’ longest serving player, will remain at the club for 2013, after scoring 11 tries in 21 appearances this season.
Former Wakefield back-rower Spicer has rejected more money from a rival club to stay with Dewsbury, and they have also retained the services of former Leeds academy prop Jones.
Jones, 23, told the club’s official website: “I’m delighted to be extending my stay at the Rams. I believe the set up at the club to be up there among the best in the Championship and I am looking forward to the new challenge that lies ahead.”