No worries! The B984 motor is the two-speed version of your old B843 motor. You will have no issues at all and you will save on your electric bill when it is running at low speed.
I did use new seals/orings, all the controls are wired up.
I have a Pentair intellitouch load center. I added a 2-speed relay to the load box, added another wire to the motor whip, and configured 2-speed operation from the remote.