Version upgrades and modernisation for businesses already live on Pantheon, done so the customisations survive the move and the books never go dark.
Businesses sit on old versions because the upgrade is frightening, not because the old version is good. What makes it frightening is never the core product — it is everything bolted onto it over the years. Custom reports somebody wrote and left. Document types that encode a process nobody has written down. Forms, and integrations built against exactly the version they are running.
So the upgrade starts as an inventory rather than an installation: what has been customised, what depends on it, and what the new version changes underneath it. Then it is rehearsed on a copy until the rehearsal is boring, and only then scheduled into a window the business can actually afford. Downtime is measured in money here, because this is the system they invoice from.
The other half of the job is knowing what not to carry across. An upgrade is the one moment a company will agree to drop the workarounds — the manual export, the spreadsheet that quietly became load-bearing, the report that three people maintain because none of them trusts the other two. Moving those forward unexamined is how an install ends up needing the same upgrade again in four years.