Starting a few decades ago, a vowel shift from [i] to [ei] has been taking place among speakers of Taicheng (台城, the capital of Taishan). It is not that all the [i] sounds have disappeared -- you can still hear them today as the shift is on-going and the process is not yet completed. Sometimes you can hear both [i] and [ei] from the same speaker depends on the the particular word uttered or whether the syllable is stressed. The driving force seems to be the less effort required to utter [ei] relative to [i] (any other suggestions?), a slack off of sorts.

Here are the rules of the vowel shift:

The shift does not affect closed syllables (the vowel is followed by a consonant). For open syllables:

1. [i] is retained with the following initials:

The alveopalatal series ts, ts^h, ɕ (dz, ts, and s in my notation), i.e. 猪 pig [dzi33], 迟 late [tsi22], 书 book [si33].
The approximant [y], i.e. 易 easy [yi32].

2. With all other initials, it can be either [ei] or [i], i.e. 你 you [ni/nei33], 姊 older sister [di/dei55].

Interestingly, the exact same vowel shift occurred (and thought to be completed by the early 20th century) in Cantonese. It is said that Yau Ma Tei (油麻地) in Hong Kong was known to be Yau Ma Ti when the British just arrived. It is probably also the reason why the last name 李 is Lee instead of ... Lei or Lay?