The upgrade kits were ordered by dealers as an exchange item. If the dealer failed to return all the old parts promptly, the exchange would be converted to a "buy outright", costing the dealer many times the original price. There would have been the occasional disorganised Apple Authorised Service Provider who found out about this policy the hard way though.
Don't forget that at the time that these upgrades were popular, there were some clone machines around that required a set of original Apple ROMs to be "legal". The Oubound was a good example http://en.wikipedia.org/wiki/Outbound_Laptop. Most people simply burned copies of the ROMs to EPROMs, but this breached copyright laws. This made Apple very careful about making sure old boards were returned.
In later years, they were not so careful. In the case of conversions from the Centris 650 to the PowerMac 7100, my employer at the time got stuck with a couple of sets of old Centris parts due to an incompetent tech ordering the kit under the wrong part number (twice!). Each cost us AU$2000 - not small change! In the first instance, we bought a case and power supply, scrounged cables, disk drives etc and made ourselves a Centris 650 out of the parts. It had no serial number. It'd be weird if a "frankenstein" machine like that ever turned up on eBay, advertised as collectable! In the early days, this would not have been possible, due to Apple having a weird policy re not selling cases separately.
Don't forget that at the time that these upgrades were popular, there were some clone machines around that required a set of original Apple ROMs to be "legal". The Oubound was a good example http://en.wikipedia.org/wiki/Outbound_Laptop. Most people simply burned copies of the ROMs to EPROMs, but this breached copyright laws. This made Apple very careful about making sure old boards were returned.
In later years, they were not so careful. In the case of conversions from the Centris 650 to the PowerMac 7100, my employer at the time got stuck with a couple of sets of old Centris parts due to an incompetent tech ordering the kit under the wrong part number (twice!). Each cost us AU$2000 - not small change! In the first instance, we bought a case and power supply, scrounged cables, disk drives etc and made ourselves a Centris 650 out of the parts. It had no serial number. It'd be weird if a "frankenstein" machine like that ever turned up on eBay, advertised as collectable! In the early days, this would not have been possible, due to Apple having a weird policy re not selling cases separately.