Is it valid...

[The Commodore]

...to swap bases with a human ally in a PBEM? I know it's just a expliot with the AI, but can I do it with a human?

[big_canuk]

It ok to do, but there is really no way to do it other than forgoing commerce income for 1 turn by declaring vendetta, and then conquering the bases from each other, and then redoing treaty or pact. Because units are removed from territory on cancelling of a pact, often it can take longer than 1 turn.