There have been alot of good answers already. If the wire is a #14 keep the 15 amp breaker in place. Technically according to the code book from what I remember a 15 amp breaker or any breaker is only suppose to be loaded up to 80%.
I hear lots of older wire is from a more pure copper source and even a better conductor than what the copper clad wire the sell today but still can be over loaded easily. Sadly most of the good pure copper did not have a ground wire.
So, not 15 amps but 80% of 15 is safe. My math is rusty but I come up with 12 amps safely on a 15 amp circuit. I would see if I could eliminate anything off that circuit at least possibly just at night while the humidifier is working.
You just never know what kind of birds nest you can be getting into when it comes to wiring especially when alot of home owners that have little experience start trying to be mister fix it. And/or a Mr. prideful green Electrician that merely thinks he know how to wire something just because it works (for a while). lol Actually that’s not really funny!
Please be extra careful. The National Electrical Code book is actually put out by the National Fire Protection Agency for a good reason :-)