A bank or trust company must use section 29.2 of this Part to compute net profits for all calendar years beginning on and after January 1, 1991 and may use section 29.2 of this Part for all calendar years beginning on and after January 1, 1990. Whether using section 29.2 of this Part for the computation of net profits on or after January 1, 1990 or January 1, 1991, a bank or trust company may recompute retained net profits for one or both of the two calendar years immediately preceding 1990 or 1991, as the case may be, using section 29.2 of this Part. Once a bank or trust company has elected to compute net profits or retained net profits for a particular calendar year using section 29.2 of this Part, net profits or retained net profits for all subsequent calendar years must also be computed using section 29.2 of this Part. If a bank or trust company has elected to use section 29.2 of this Part to compute net profits or retained net profits for a particular calendar year, it must thereafter use section 29.2 of this Part to compute retained net profits for that calendar year.