I have not made any additional contributions, but the individual stocks have changed over the years. Is it all still considered inheritance, or is this determined by the accumulation date of each stock being before or after the inheritance date?
The portfolio is not the inheritance. The inheritance is the specific shares that were in the account on the date of the previous owner's death. Anything that you purchased after the date of death is not inherited. When you sell shares that were in the account on the date of death you can designate them as inherited.
you may no longer own any of the inherited shares.
you must keep good records of date purchased and price, as well as reinvested dividends, if any.
If you have stayed with the same broker all this time, the broker has the records.
after 12 years, the "inherited" designation doesn't help you .