The effective mass of a tone arm is effected by the location of it’s counterweight. Increasing the distance of the c/w from the arm’s bearings increases the arm’s effective mass. To decrease an arm’s e/m, add weight to it (wheel-balancing stick-on lead weights work) and move it closer to the bearings. To increase it of course do the opposite.
The Groovetracer company makes an excellent after-market counterweight for Rega arms, and it comes with an assortment of different screw-in weights. That allows the same arm to be used with a variety of pickups featuring different suspension compliances.