If I die before some sort of financial shock comes along which causes me to cash in I Bonds, they are registered as POD, payable to my spouse on my death. I believe the I Bonds would not be involved in probate, if my wife survives me.
If I wanted to register somebody other than my spouse as POD, I would talk to a lawyer familiar with marital property rights (I live in a community-property state), about obtaining her consent. I do not expect that I will want to do this.
If I survive my spouse, I believe it would be possible to change the POD registration to other individuals.