Aug, pg.12
If you want legal ’ware, bodyshop chains are your best option
for speedy and efficient implantation, especially if you’re looking
for a minor enhancement or an upgrade to existing implants.
Aug, pg.128
Essence holes are important when replacing or upgrading
implants. If a character replaces an implant with one of a higher
grade, the new implant may well have a lower Essence Cost than
the old one and the character will develop an Essence hole that
she can fill with further augmentations. Likewise, upgrading an
implant often increases its Essence Cost
As I said before, things are worded horribly (as normal for Augmentation), but there is at least as much support for being able to upgrade implants rather than flat out replacing them. The key area of haziness is actually the way they use the word upgrade. Half the time it seems to indicate an increase in rating, while the other half the time it seems to indicate an increase in grade (similar to how Augmentation uses Normal as both a grade, which its not, and a classification of bioware). Those who don't think ware should be upgradeable most often point to these sources:
Aug, pg. 128
Most augmentations, whether bioware, cyberware, genetech,
or nanocybernetics, have an associated Essence Cost. When
a character has an implant removed to be replaced or upgraded,
Aug, pg.32
While most second-hand cyberware surfaces when former users
upgrade implants and sell off their old ’ware,
I take the later to refer to upgrade as in putting in a higher grade of ware (ie replacing your normal Muscle Toner with Alphaware Muscle Toner). For the former, a careful reading will show that it never actually says that old ware is removed and tossed out then completely replaced with new ware (that is considered an upgrade). It states that the implant is removed to be upgraded. All it shows is that when upgrading where it has to be removed, then upgraded, then slapped back in (meaning that you have to go through both removal surgery and implantation surgery still for the upgrade).
Keep in mind as well that they don't refer to getting an upgrade (meaning an upgrade for the character) but instead refer to upgrading the implant itself (making the implant better). As I pointed out before, even without being able to upgrade ware, RG still isn't a good deal. You end up paying a high premium on the item just to have it now and, more importantly, using up 1/7th of your available Postive Quality points on a piece of gear that will, most likely, get replaced within a couple dozen runs.