Does the cost of cheek implants increase if I want to get a size upgrade from the ones I have already?

The cost to remove your current cheek implants and replace them with a larger size would really be at the discretion of your surgeon.  It is possible the cost could be similar to what you paid to have the current implants placed or it could be more.  

 Hi, I have performed many Cheek Augmentations with cheek implants to shape the cheeks for over 30 years.  The cost for cheek implant removal and replacement (at least in our office) increases over primary placement cases because they are more difficult to perform.  The tissue is more difficult to dissect, the existing cheek implants must be located and removed gently (so they remain intact and don't leave a broken segment inside the tissue), the implant pocket must be developed especially if the existing cheek implants are too small. 
 For all of these reasons we charge more for revision cheek implant cases than for a primary cheek augmentation.  Whether the cheek implants are size medium or large and 3 or 4mm does not affect the price.  Specific costs can only be provided once a thorough history of what was done previously is obtained as well as existing in person or photos and an electronic consultation has been completed.
Hope this helps.