It's a common misconception that if you can play one keyboard instrument well, you can probably play another well. This would certainly not apply to the piano vs the harpsichord.
Harpsichordists can barely get piano keys down; pianists bang like hell on harpsichord keys. What you've spent many years learning to do reflexively cannot in an instant be unlearned.
It takes no strength in the hands to play the harpsichord. It's played with the fingers only, that is, the only role for anything behind the top knuckle is to position the fingers over the keys -- no weight from the hand, arm, shoulder, or back -- nothing beyond what it takes to get the key down, which is very little.
The piano, on the other hand, takes real strength to play, the entire weight of the back, shoulder, arm, hand, and fingers must be brought down into the keys.
But the big difference is this: with the piano, how you get the key down is everything, since it controls the rate at which the hammer rises to strike the string. How you let go of the key is relatively much less important. In addition, a pianist never really touches the string directly with the hand.
With the harpsichord, it's the other way round. As the key does down, your finger comes in contact with the string through the mechanism; you feel the string stretch and finally release as the plectrum plucks it.
It makes almost no difference in volume or tone whether the key goes down quickly or slowly; how hard you play only barely affects the character of the sound. But the sound rings on an amazingly long time, until you let go of the key, which you do very carefully and with very independent fingers.
In sum, on the piano, the action is downward from the seat of your pants, using the whole upper body, into the keyboard. On the harpsichord, the action is upward, out of the keyboard, using your finger from tip to the top knuckle.
Learning how to play a keyboard instrument is a long process of training neural pathways between your eye, ear, hand, and spine (the brain is not much involved; we can't think fast enough to control such an action anyway). Unlearning many years of the downward motion and replacing it with the upward one is real work and also takes many more years. It seems to me this work is never really finished.