Would it be safe to replace a devices 15 amp circuit breaker with a 12 amp breaker with the same voltage ratings? Also going the other way could a 20 amp breaker with proper voltage ratings be used in a circuit rated for 15 amps safely?

 

the 12 volt replacement would be safe, but would trip more often. the 20 should not be put in since it would trip 5 amps later than the 15.

 

you can downsize, you cant upsize unless the wire connected to the OCD can support the up-sizing, etc. and, also depends on what the OCD is protecting, thus up-sizing, even if the wire can support it, may cause components to die, etc.

 

As noted, you cannot safely upsize the breaker without a lot of research. You would need to have full knowledge of every protected device on the circuit - every wire, switch, outlet and so on. Where is the weakest link? The SAFE design makes the breaker the weakest link.

A qualified electrician that is familiar with local codes and how they have changed, and how your home is likely wired, MIGHT be able to comment on whether going to a larger breaker is possible. Running another circuit is usually the batter option, if you need more juice somewhere.