replacing tonearm question?

Because the cartridge has rather extreme leverage over the bearings it would take a pretty bad bearing to dislocate the cantilever one way or the other but the "noise" from a rough bearing can be transferred down the arm to the cartridge. Tap lightly on your tonearm tube towards the bearing end. If you can hear it you can also hear your bearings if they are not absolutely smooth. 
Another fun experiment to determine how well your turntable is isolated. Keep the turntable off and place the stylus down on a stationary record. Turn the volume up and listen to what you get. It should be dead silent other than the noise from your phono amp. If the cartridge is too stiff for the tonearm it will start feeding back wildly. Too loose and the tonearm will start bouncing. All this just from the energy in the room. 

Simao, it all depends on the geometry of the tonearm's and their base configuration. You have to take the Sumiko off and look at the hole size in the arm board. If the Jelco requires a bigger hole or a slightly different position to get it's overhang right you will have to enlarge the hole then see if you can mount the Jelco without an ugly result. The cleanest way to do things is always by getting a new arm board which I am sure VPI can supply. You can also make one yourself by using the old board as a template. If you do not have the capability to do this any wood worker with a drill press can do it for you.