A changeover note is a note from a prefix or series of ten million notes (usually) in which an important change occurred, often a change of signature, or some modification such as the change from the 1954 Devils Face to the 1954 Modified portrait.
For example, the 1986 $2 series with prefix CBH is a changeover series. Notes numbered CBH 0000000 to CBH 6719999 were signed Thiessen-Crow, and those numbered CBH 6720000 to CBH 9999999 were signed Bonin-Thiessen. The actual changeover (in signatures) occurred at note number 6720000 but all notes with prefix letters CBH are called changeover notes in collector jargon. Notes from the previous series (prefix CBG) are all signed Thiessen-Crow, so they are not changeover notes; neither are those from the next series, CBI, which are all signed Bonin-Thiessen.
The -i in a catalogue number is for distinguishing quite small differences, often in the printing company which made the notes. The numbers, such as BC-55, identify major designs / denominations; the following letter a, b, c etc. is used for lesser differences such as signature varieties, and -i, -ii etc. for very small differences.