Have done a couple of Illumination to QS upgrades last year.
Both were large systems and went very well.
Lutron has a ton of info about what works and what doesn't, or will require extra bits / licenses etc.
Having the old program of course is ideal, but on a smaller project starting fresh should not be a big deal.
This is also an opportunity to let the client know about things they may not have had in the old system and might want to add to the new.
New model keypads and Occ sensors are a couple of popular things.
One of the upgrades I did went from old Seetouch KP's to Palladiom, and there were 50 of them (someone here will do the math).
If you did not do the old system, you may have a different programming approach that the client will like.
Finding an old Homeworks processor and saving the client money today is fine if they really do not have the budget to upgrade, and you still save the day by getting the system back up and running. But, they still have a system that is old and will eventually require you back to upgrade.