is it legal to change benif. on life insurance policies of elderly when it had been established for many years just b4 passing?

It is legal for the owner of the policy to change beneficiaries at any time during their life.  If someone else changes the beneficiary, they must have authority to do so, either through a Power of Attorney or court-appointed guardianship.  However, the court is wary of people who change it to benefit themselves, especially when done shortly before death or around the time a person becomes incapacitated.  It's something you should discuss with an attorney, as there are ways to reverse it, if certain circumstances are in play.