For people traveling to Israel during a time when those in Israel are already saying v’ten tal u’matar livracha, but we in chutz la’aretz (the diaspora) are not yet saying it, does one keep one’s diaspora practice in Israel or adopt the Israel practice for the few days that one is there?
The Mishnah Berurah (MB 117:5) says to keep practices of the place from which you originate, and not say it.
However, for my part, I cannot see being in Israel, where and when they actually need rain and are praying for it, and not identify with the people around you—Jews who are connected to land and its produce—and rather identify with generic chutz la’aretz people, to whom you are not really connected, and whose need for rain is anyway not reflected by the times we do or don’t say v’ten tal u’matar. In other words—in Israel the need is real, these are our people, and v’ten tal u’matar is actually synced to when rain is needed. In chutz l’aretz—not so much. It all just follows formal rules. This is the position of the Chida (Birkei Yosef OH 117:5), Rabbi Betzalel Stern (Betzel HaChochma 1:62), and Rabbi Shlomo Zalman Auerbach (Halichot Shlomo, Tefillah 9:22).
Based on this reasoning, I would say v’ten tal u’matar if I were in Israel during this time.