In my parents’ home we always said Kiddush standing. I continued this tradition for some years after getting married. I then changed to saying Kiddush sitting (I do not recall the reason and neither did I do hatoros nedorim). Am I able to go back to my original custom of standing for Kiddush.
I assume that the question refers to Friday night Kiddush.
Because there are different customs concerning Kiddush on Friday night, and there is no clear-cut halachic decision, it is permitted to return to the original custom. As Iggros Moshe writes concerning the nusach of prayer, changing from a family custom is more of an issue, but going back to a family custom is fine.
However, if one began to sit because of a halachic concern (for instance, to fulfill the Mishnah Berurah’s recommendation, based on Vilna Gaon, to sit), one should perform hataras nedarim.
Sources: According Tosfos (Berachos 43a) people should sit for havdalah in order for the listeners to fulfill their obligation through the recitation. The Vilna Gaon cites Tosefos’ comments and adds, “and this is correct,” and although it is unclear if this is a reference to the conclusion, or to the justification Tosfos gives for standing, Mishna Berurah (271:46) writes that in the Gaon’s view, one should preferably sit (see also Shaar Ha-Tzion 51). He concludes that one should preferably make a point of sitting.
On the other hand, Rav Chayim of Volozhin testified to the fact that Vilna Gaon personally recited havdala while standing, and Arizal writes in Sha’ar Ha-kavanos that one must stand during kiddush.
The straightforward reading of the Shulchan Aruch (271:10) is to stand while reciting vayechulu and to sit for the second part, while Rema writes that “the custom is to sit even while reciting vayechulu,” writing that one stands a little at the beginning in respect of the Divine Name alluded to by the opening words. Rema also cites Kol-Bo, who writes that one may stand or sit, though preferring sitting to standing.
There are different minhagim (sitting, standing, or half-half), and because each has a halachic source, one should follow his family custom. Because there is no clear halachic preference, it is also permitted to return to one’s original custom.
Yet, if there was intention to follow a halachic stringency by sitting, hataras nedarim would be required. If there is nothing else, one may use the custom becoming a neder as a petach for the hatarah.