how to get old and new coverage amount in "Enrollment Certification Required" formula

We want to make EOI required when the coverage amount is more than 150K or if increment i smore than 10K.

so we need to have

1. Old Coverage Amount
2. New Coverage Amount
3. increment = (new coverage amount - old coverage amount)

if ( increment > 10000 or new coverage amount > 150000) then

return Y (means EOI certification required)

L_BEN_CVG_NEW_AMT = ben_fn_get_char_value('BEN_PRTT_ENRT_RSLT'
, 'BNFT_AMT'
, 'Voluntary Life/AD&D Employee'
, 'Voluntary Life/AD&D Employee')

l_day_before = add_days(l_eff_dt,-1)
change_contexts(EFFECTIVE_DATE=l_day_before)
(
L_BEN_CVG_PREV_AMT = ben_fn_get_char_value('BEN_PRTT_ENRT_RSLT'