Matching on names
If the two systems are matched to each other by customer name, a handful of records will attach to the wrong person, and nobody finds them for months. Every record should carry its old system identifier so a row on one side can always be proved to be the same row on the other.
Your customers hearing about it
The new system will happily send confirmations, reminders and cancellation notices while you are testing. Every outbound message has to be switched off at the account level before anything is touched, and every operation carries its own suppression on top of that. Healing Waves rebalanced thousands of records across two weeks with zero patients contacted in error, and that was the result of two independent switches rather than care.
A calendar that looks right and computes wrong
Capacity rarely maps one to one. Four treatment tables, three crews, six units, two bays: the new system may model that as one resource and start turning work away while the screen looks plausible. This has to be checked against what the system thinks it can accept, not against what it displays.
The old system kept working
It took bookings and cancellations all the way through the move, so the export you started from is already out of date. A second export and a proper comparison is not optional. Read only first: cancelled, moved, mismatched and never migrated look identical in a plain diff and need opposite responses.
The website, if it moves too
Every link anyone ever shared, every bookmark, every directory listing and every indexed page points at the old addresses. Without permanent redirects they all become dead ends on the day you switch, and the search authority built over years goes with them. Fifteen redirects took an hour on the Healing Waves build. After the fact, it cannot be recovered.
Email, when the domain is touched
Mail usually runs on the same domain settings as the website. Moving the domain wholesale to a new host is the common advice and it puts mail delivery at risk to gain the website nothing. The rule is that the domain stays where the mail is, and only the records pointing at the site change. Then you check afterwards rather than assuming, because detaching a domain from a host can silently remove records nobody was thinking about.