Erm.. last I checked you can't change your name "in" tf2.. Although there was something in the changelog about not requiring the overlay I'm pretty sure it will still take it from the community.
The term steamId is a bit ambiguous because valve started using the term to also mean the name of your community profile. i.e. the constant name that represents you on the community. My steam username is TheRealZogger (as zogger was taken), while my steam community id is Zogger (I signed up in the beta of the community so those pesky zogger imposters didn't get to register before me). My original steam id is a number as described in oL's post. My display name on steam community is also Zogger but that is entirely different to all of the above. You can change this at any time and it will be used as your in game name in TF2, as well as the name that's displayed on yours / other people's friends lists.
The term steamId is a bit ambiguous because valve started using the term to also mean the name of your community profile. i.e. the constant name that represents you on the community. My steam username is TheRealZogger (as zogger was taken), while my steam community id is Zogger (I signed up in the beta of the community so those pesky zogger imposters didn't get to register before me). My original steam id is a number as described in oL's post. My display name on steam community is also Zogger but that is entirely different to all of the above. You can change this at any time and it will be used as your in game name in TF2, as well as the name that's displayed on yours / other people's friends lists.