pg_upgrade: can I use same binary for old & new?

Hi,

[Hope this gets through after dumping DKIM-ignorant mail provider.]

Wanting to upgrade from:
PostgreSQL 15.13 on x86_64-mageia-linux-gnu,
compiled by gcc (Mageia 15.1.0-1.mga10) 15.1.0, 64-bit
to:
PG 17.5

Way back, I was able to use -k|--link option on pg_upgrade (PG13 to PG15);
but since then:

- my DB has grown to over 8TB
- even with ~70TB, I don't have enough contiguous disk space to
dump/restore
- my Linux distro (Mageia) is not setup to handle multiple versions of
postgres (installing 17.5 removes 15.13). Worse, it failed to install
part of the module when it saw /var/lib/pgsql/data still there:
https://bugs.mageia.org/show_bug.cgi?id=34306

I've glanced at the pg_upgrade source code (my C skills are ancient) and
it appears pg_upgrade is virtually the same from 15.13 to 17.5.

My question: did I miss anything, or would:
$ pg_upgrade -d data15 -D data17 -k
suffice?

Besides not noticing significant difference between the two versions, the
docs at https://www.postgresql.org/docs/current/pgupgrade.html
contain: "default is the directory where pg_upgrade resides"

If new pg_upgrade is the only binary, will both -b and -B default to it?
Maybe at minimum I may need to specify:
$ pg_upgrade -b /usr/bin -d data15 -D data17 -k
?

Thanks,
Pierre