Cancelling "vacuum full" in single user mode?

Hi,

I have a customer approaching transaction wraparound, about 3million
transaction IDs away at the moment.
Postgres 9.5 (yes, I know...)

Somewhat mislead by the message to vacuum the database in single user
mode, they are now in single user mode and are running "vacuum full"
on the "worst" database, which has 18.5 million tables, and is about
350GB on disk.

Now we want to cancel the "vacuum full" and run a "vacuum freeze"
instead... or preferably, start back up in multiuser mode and run
vacuum there, but that was running very slowly while autovacuum was
trying to run (so we would need to disable that too, for now).

So,

1. Can we cancel safely?
2. Any other suggestions? Upgrade to a newer version right now?

/Colin