(I think even if you use link mode, it doesn't modify original catalog files, so you can start the old one until you have started the new one. I'd look for more info from someone more knowledgeable about this exact process.)
Correct. I've had it go wrong with link mode. As long as you don't start the new cluster, the old one still works absolutely fine with a minor (documented) step.
IIRC, the data files (in /data/base) are linked (and unchanged by the process). The control files, transaction logs, etc aren't, but the old control files are "disabled" to prevent accidental starting of the old cluster (it's easy to re-enable it)