The Sefer Ashrei Haish quotes Rav Elyashuv zt"l who says that one who has the Minhag of not eating Gebrokts may change his Minhag to eating Gebrokts.
It is preferable to make Hatoros Nedarim but not necessary. One may rely on the Hataras Nedarim made on Erev Rosh Hashana.
Reb Elyashuv holds the original Chumra of Gebrokts started when Matzohs were thick.
Today there is no chance of having unbaked flour in our thin Matzos.
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)