Butt Implant Too Small

Yes, Butt Implants, like breast implants can be exchanged for larger ones in most cases..One other option is  Fat transfer for Buttock Augmentation. It  can be done at the same time and afterwards as a single session or more to improve the size and shape of your buttocks. 

If legitimate semi-solid silicone rubber buttock implants were used then the implant itself will not shrink or flatten.  However, it is possible to remove and replace the current implants with larger ones now that the tissue has had a chance to stretch out and relax since the last surgery.  Additionally, you may want to consider Hip implants too to give you those "curves" you are desiring.  Hip implants are inserted through a 1 1/2 inch long horizontal incision just below the belt line (within the bikini line).  The implants are made of the same material as buttock implants therefore they are permanent and cannot leak or rupture over time.  Best of luck... drryanstanton.com

Yes it is possible to upgrade in size, but remember to be cautious; the larger the implant, the more possibility of complications such as sermons or implant extrusion.  The safer option is to try to have fat transfers over  and around the implant to get the shape and size you want, since you are considering fat transfer to the hips.  Good luck!