The Rebbe's guidance on nusach (the version of the prayers) and prayer customs:
Nusach Ari suits everyone. Nusach Ari, as arranged by the Alter Rebbe, is fitting for every Jew and is halachically sound as a nusach for all (IK 3445), with special spiritual advantages (IK 4012). Praying in Nusach Ari is a core Chabad practice (IK 3021), and a Lubavitch shul should use it (IK 4350, 5675).
Switching nusach: only for good. Changing to Nusach Ari is proper when it is a firm, lasting decision (IK 3569, 4563, 6332, 6704, 6721, 7107, 8480, 9687). One may switch toward Nusach Ari but not away from it, and should not alternate between versions (IK 7035, 7039, 8480). Someone who adopts Chabad customs should also daven Nusach Chabad (IK 9408), and it is proper for a yeshiva student to adopt the Chabad nusach and tefillin knot (IK 7012). Continuing a father's nusach and customs brings great merit (IK 2355). Students shouldn't be pressured to change their nusach (IK 8668).
Peace comes first. Keep your own nusach and customs, but without causing disputes (IK 5165, 5788, 7032). When leading services in a shul with a different nusach, if your own would cause conflict, peace takes precedence (IK 3174, 1303). Spread Chabad ways by personal example rather than confrontation (IK 7036). Even when shuls unite, each should keep its own nusach (IK 5683), and a single unified prayer text for all communities is not the way (IK 9049).
Customs and texts. Where the Alter Rebbe's Siddur and Shulchan Aruch differ, follow the Siddur (IK 3976, 8649). The Chabad Siddur includes only prayers relevant to everyone (IK 7406); Chabad does not recite the Thirteen Principles daily, since they are not in the Alter Rebbe's Siddur (IK 3823). One may keep the Sephardic pronunciation one is used to (IK 7462).
See also: Prayer – Praying with a minyan and shul life.