The letter at the beginning of the HKID number normally has no meaning at all: it's just part of the number. The "symbols" referred to in the link above are those in the area labelled "Symbols" in the diagram.
The only exception to this is that Imported Labour and FDH Visa holders who arrived after December 1995 can be identified by a HKID number starting with "W". See Q7 here:
http://www.immd.gov.hk/ehtml/topical_8.htm
Otherwise, your HKID number is yours for life even if your immigration status changes (e.g. you get PR after 7 years).