Although you may get away with it just the once, it is a risk that the membrane(s) could get damaged. My answer would be that if you really must purify some more water before replacing the pre-filter, use resin only. Expensive but if it's only for one tankful then it shouldn't break the bank.
As for not changing the pre-filters in 18 months, I really don't know how Richard01903 has got away with that.