First, breast feeding will not likely be a problem with breast implants in place. Breast feeding will not affect the position or the size of youre breast implants themselves. However, the overlying breast tissue may well undergo changes after you complete nursing. These involutional changes can result in loss of breast volume with accompanying sagging due to excess skin. If this is significant enough (and the effect cannot be predicted) then some patients do reqest some form of lift to reshape breast tissue over the implant.
Many women never require any additional procedures after nursing. The consequences of shape change will depend on the amount of involutional loss of breast volume and the elasticity of your skin. These changes are probably genetically linked.