Changing from sect to sect does not require anything else besides declaration of intent and following the practice of the new sect. This conversion takes place all the time. It has been taking place for 1400 years.
On a large scale, it happened in Iran in the 18th century when their Shahs converted and it happened in the 19th century in the south-eastern provinces of Iraq.
On an individual level:
• It is a common practice for people to become Shiite when moving to live in a predominantly Shiite area or vice versa. It happened constantly for the past 1400 years. It happens all the time today.
• In the Shiite doctrine, if someone dies leaving only daughters, then his inheritance goes completely to those daughters. In the Sunni doctrine, the person's brothers get a share. This has been a frequent cause for conversion for such people – mainly in the cities. [One notable case that comes to my mind is a member of the now-defunct Governing Council who was generally regarded as "representing" secular Sunnis. This gentleman has only three daughters and has converted to Shi-ism.]
• In Islamic marriage, the dowry is in two parts; one part is paid to the wife in advance. The second part is called the deferred dowry. In the Sunni doctrine, this is paid in the case of divorce or death, whichever comes first! In the Shiite practice, this has to be paid on demand to the wife, at any time of her liking! In practice, this is hardly an issue as failed marriages are few and far between. In mixed marriages (which are numerous, especially in "mixed" areas) this question comes up and has to be agreed upon. In such marriages, there is no requirement for any of the partners to convert. The difference in sect between husband and wife is a constant source of family humor.