Since I already have implants, what are my options for lumpectomy?

Typically, the answer here depends on your breast size and the level of concern regarding the lesion to be removed.  Most commonly, you will have this procedure done first and then see about changing the implants out with or without a lift.  You do not need either implant changed unless there is a reason to do so.  Often if one is exchanged, the other is done at the same time.  

Your best option is a lumpectomy design that results in a lift type procedure.  The implant can be exchanged or left in place if possible.  At a later date, maybe after radiation, the other side can be treated for symmetry.  

This is a complicated issue and you will likely get very different answers from different plastic surgeons. The concern with breast implants and lumpectomy have more to do with the radiation therapy that follows than the lumpectomy surgery itself. A lumpectomy or partial mastectomy is almost always followed by radiation therapy. The delivery of that radiation depends upon the tumor type, grade, size, location, etc. When whole breast radiation is given, the breast, the implant, and most importantly the implant capsule are affected. Capsular contracture rates can reach as high as 80% in post radiated patients. A capsular contracture is a tight scar capsule around the implant which leads to implant deformation, firmness, and possibly pain. When this occurs, the operation to treat the contracture has a high complication rate for infection, seroma, bleeding, skin loss, etc. This is such a problem that many plastic surgeons recommend a mastectomy (preferrably nipple sparing) if it can avoid whole breast radiation. However, so patients are candidates for partial breast radiation or balloon (SAVI) radiation which does not have the same impact on capsular contracture. I have had success using acellular dermal matrix (e.g. Alloderm, FlexHD) as a capsule replacement prior to radiation in preventing contracture. As for replacing the implant on the other side, it depends on how it looks, how old the implant is, desire for any improvement in appearance, or whether the cancer side will require a site change (switching from subglandular to submuscular). As you can probably tell, this is a complicated subject. Your best option is to visit with plastic surgeons who work routinely with your oncologic breast surgeon and find out what they recommend.