שלושים יום קודם החג
According to Rav Elyashiv zt"l, one who has the Minhag of not eating Gebrokts
may change his Minhag to eat Gebrokts. It is preferable to make Hatoros Nedarim
but not necessary. One may rely on the Hataras Nedarim made on Erev Rosh Hashana.
R.Elyashiv holds, the original Chumra of Gebrokts started when Matzohs were thick.
Today there is no chance of having unbaked flour in our thin Matzos and there never
was such a Gezeira on thin Matzos.
Similarly, if the in-laws eat machine matzohs one may also do Hatoras Nedarim
and eat Machine Matzohs.
For Kitniyos one may not change, Hataras Nedarim won't help. Even an Ashkenazia
who marries a Sephardi (follows husbands Minhagim) has to keep her original
Chumra and should not eat Kitniyos.
The Chumra of Kitniyos is different than all other Chumros. Kitniyos was a Kabala Gemura
till the end of all generations. (Ashrei Ha'ish 3)