If an individual is granted Letters of Administration upon an Estate that the original Executrix could not fulfill, can the new Administrator change the will according to how they see fit?
Once an individual dies, their will cannot be changed.
No. The Last Will and Testament is the decedent's final statement how his/assets should pass to his/her beneficiaries, and the Executor is bound to follow the terms of the Will. There are issues that may impact upon the Will though (i.e., a spousal right of election, Will construction if the terms are not clear, building in Special Needs Language, etc).