You could register the new nick (I see that it is still up for grabs), update your current bio to say that it is retired, and link to the new home node.

That's exactly what I'm planning to do - for my own reasons, I've been getting rid of all my "Screamer" accounts wherever feasible. I've given it some thought and decided to reach saint on the Screamer account (I'm vain like that :-)), then register a new, permanent name, leave a link to my new homenode on the Screamer homenode, put a link back to the Screamer homenode into my new one, and carry a remark to that effect in the new account's sig for a while.

Using the two accounts to cross-upvote didn't even occur to me, and seems hardly worth the effort. The real problem I thought of was people referring to my posts. Being able to change nicks would break the "integrity" of other people's references to me by the nick "Screamer" or links to the Screamer homenode, f.ex in reply to a note of mine, or esp so when referring to a note of mine in a completely different place in that or even a different node.

So long as you are not using the accounts concurrently, but instead, entirely abandon the old one and use only the new one, I don't see any kind of ethical implications.

Makeshifts last the longest.