Assuming your employer now uses the normal method of calculating your holiday entitlements (and that method satisfies the requirements of the statutory minimum holiday rules) you should lose two fifths of your TOTAL holiday entitlement- that is you should lose two fifths of your company holiday entitlement and two fifths of your bank holiday entitlements. This assumes all your days will be of equal length.
Check how your company calculates your actual entitlement. The normal practice now is to calculate it by adding together company and bank holidays. If as a full timer you get 27 days now + 8 bank holidays = 35 days, you would lose two fifths of that leaving (14 days) you with a total of 21 days.