Is There a Sizing Difference when Switching Saline to Silicone?

When changing from saline to silicone, size for size volume they are the same. Going from 350-500 will probably add a cup size of volume.

Removing old breast implants (especially the capsule you alluded to as "scarring issues") enlarges the pocket for subsequent breast implants making the same volume implants smaller. With time, there is loss of volume of breast tissue meaning that larger implants would be required to compensate for it.
On the other hand, the larger and the heavier the implants, the more they stretch the breast leading to loss of tissue and sagging. Therefore using ever larger implants rapidly cause breast sagging and irreversible breast tissue changes.
The solution? Use temporary sizers during surgery which would give you the clearest idea of which EXACT implants would give the best appearance without necessarily just placing a pair of large implants in and requiring another procedure in 1-2 years.