When you're considering moving a soil grown plant into a hydroponic environment, start by considering the size and location of your plant. Annual seedlings are pretty easy to move - it's pretty much a matter of rinsing the soil and other matter off the roots and placing them into your system. Be sure to provide some additional nutrients to help the plants recover quickly from transplant shock.
Plants that are well established, like potted perennials, have more involved root systems and moving them will require more care since your goal is to minimize stress and damage to the roots. You'll also need to account for the amount of space occupied by a potentially large root system, although once the plant is settled into the hydroponic environment, some amount of pruning may be possible. Larger plants that have been growing freely and are rooted into the ground are not good candidates for switching to hydroponics - excavating them from the ground will inevitably lead to root damage and put them under significant stress.
