well yes you could - with a deed of variation apparently
It
http://www.ts-p.co.uk/uploaded/publications/information_sheets/Inheritance_Tax/deeds_of_variation.PDF
it looks pretty horrendous.
I think it is obvious that the will you are administering was drawn up with the expectation that lawyers would be involved somehow
I think you are learning the hard way that testamemtary trusts 'may have had their day'.
I mean questions like: can I successfully issue a deed of variation altering a testamentary trust if there are as yet unborn beneficiaries ? is not really the stuff of Answerbank. [ altho I think the answer is no as all beneficiaries have to agree ]