Kotarashi said: But since it's a gender swap, Terra, sounds best, IMO.
I generally follow one of two camps on the subject: 1. Make the least change possible to the name to make it reflect the opposite gender. Terri is a more normal name than Terra, and doesn't change the sound, so Terry -> Terri wins.
2. If the name is in Japanese, use an alternate, more feminine reading for the characters whenever possible; if not, either add -ko (inelegant solution) or use the closest equivalent name.