Oracle: Can I change the oracle's password in a RAC environment? Can I change the oracle and grid (OS users) passwords in RAC nodes? 
Changing it could break the RAC?

The Oracle Grid and Database installers asked me to input that passwords, unfortunately that was easy and I have to change it now, but there are instances running on that in production.
Yes,

you can change the passwords without any worries. 

The reason for that is no cluster configuration file stores information about OS password, hence changing it will not affect already existing user equivalence.

Also, keep in mind that passwords do not need to be identical on both RAC nodes. It is common, but not a must.