Hi there,
Downsizing implants can be a "tricky" procedure to perform without the associated tightening and lifting of the pre-stretched breast tissue envelope. There are a multitude of breast factors, patient factors and surgeon factors for consideration before determining the most appropriate technique in achieving the optimal result. 
In essence, it is a form of revisional breast implant surgery and it's important to ascertain the reasons for a downsize. Whether it's for a functional problem (in relation to capsular contracture, implant leak/rupture etc), an aesthetic deformity (implant malposition, breast animation deformity etc), unexpected life circumstances such as significant weight fluctuations, pregnancy effects etc., or even simply a change in life style or personal preferences, defining the purpose and goals for the surgery is fundamental before deciding for the actual implant options and the associated surgical approach. Realistic patient expectations & desires are the beginning of a successful & beautiful surgical journey. 
Without seeing you in person or having any clinical information concerning your previous breast implant surgery, it'll be very difficult to offer you the most suitable surgical solution. I highly recommend you to return to your original surgeon for further in depth advise. Or else please submit a personal enquiry to my practice via PSF and my team can arrange a Skype consult for further discussion accordingly.
Hope that helps.
Best wishes
Ellis Choy