Can fully paid preference shares forfeited?

No ,In Any Circumstance Fully Paid Up Shares Can't Be Forfeited , If 1/5 is representing a conversion price or valuation based formula then it can be done,like 5%,1000 CCPS issued at Rs.10 ,Conversion price is Rs 2000 Per equity shares (1/5 th Value ) then automatically CCPS deemed to be converted ... otherwise shareholder itself don't want to take remaining 80% then Cash can be paid or rollover can be done or reduction/buyback is possible (but generally its not happened in that way )