The rules on what constitutes Passover chametz vary according to Ashkenazic and Sephardic traditions. Since we are all Jews, what really prevents those of Ashkenazi descent from eating foods such as rice or legumes during Passover? [What is the basis for following the tradition, custom, or minhag that one was brought up in, and when, if ever, may one properly change it?]

Your question is phrased somewhat incorrectly.

 

Ashkenazim and Sefaradim agree on what constitutes Hametz: any amount whatsoever of any leavened product made from wheat, barley, oats, spelt and rye. These products must not even be owned during passover, nor may one derive any economic benefit from them.

 

But different communities have different traditions about what should constitute "fences around the Torah" - those additional measures instituted by sages and communities through the centuries to ensure that there are no accidental violations of the Torah's ban on Hametz. Ashkenazim have the custom of not eating "kitniyot" (a catch-all category, not co-equal to the botanical term "legume") for a handful of reasons. Perhaps the custom began because these items (like corn) can be made into flour and used for bread-like substances, so it would be better to avoid them. Perhaps it began because these items are so small (like rice) that if any kernels of genuine Hametz would be mixed in with them in the course of marketing, and one would inadvertantly transgress the prohibition of Hametz itself. 

 

(By the way, Sefaradim are not totally unconcerned with these issues also. Sefaradim sift through their kitniyot before Passover to check for the presence of foreign matter. And in some cases - especially in Israel - they are able to buy kitniyot certified for Passover.)

 

Whatever the original reason, Ashkenazim have avoided these products for centuries. The power of "minhag avoteinu," ancestral custom, is strong in Judaism. All things being equal, absent some compelling extrinsic factor, I recommend Ashkenazim continue keeping Passover in the way their forebears did. This policy would maintain familial continuity and reduce the possibility of familial strife. And this is my own personal behavior.

 

However, it seems to me that there are some circumstances which warrant changing the custom. First, someone newly observant who comes from generations of non-observant Ashkenazi Jews, whose parents and grandparents did not observe the prohibition on Hametz, let alone kitniyot, should feel free to adopt Sefardic practice, which is indubitably cogent as Halakha. Second, vegetarians who need protein should consider eating beans and lentils, if purchased before Passover and sifted to ensure that no extraneous Hametz has been mixed in. Finally, a strong argument can be made that, in Israel, eating Kitniyot is a gesture toward Jewish unity, as the various exiles -- from Iraq to Morocco to Poland to Germany -- are now mingled again.

 

 As a reform rabbi, I am committed to the concept of informed choice. Therefore, I am free to make my own decisions. There is no gainsaying the fact that Ashkenazi and Sephardi Jews have historically developed many different practices. However, the reality is that this is in large because they were raised in different cultural environments. Today, we predominantly live in the polyglot western world. Therefore, unless one has a passionate commitment to remaining in one culture or the other, one has total flexibility in choosing from one minhag or the other. As far as I am concerned, this applies to dietary rules, naming rules, etc.