Have you looked at the switchgear install guide for these? Should show you everything you need to know. Too big to https://files.engineering.com/getfile.aspx?folder=fc829a57-e040-4435-84e6-eee338eafeb7&file=FC_SWGR.pdf
We do this type of modifications all the time, should be simple. Most 1200/2000A cells can be easily changed from one rating to the other by changing the CT's and rejection hardware (And maybe the stabs) What type of gear is it?
OFG, we see that often in some older ABB metering CT's. They will actually melt on the shelf in a warehouse.
OP, did you have something specific going on that caused you to ask this question or just a general curiosity ?
Some electronic trip units will default to the minimum setting when you change the settings (Between settings), I believe the micrologic series will do that, so depending on load the breaker may trip. Best to do it de-energized.
Air, gas, vacuum? Depending on the specific breaker type the OEM may or may not have published values. In absence of published values ANSI/NETA says that all 3 readings should be within 50% of the lowest reading.
There is nothing wrong with those breakers, Federal Pacific gets a bad rap from some issues with their residential breakers in the 80's but their LVPCB's have no real issues.