Upgrading is usually very expensive, particularly if you need to have it done professionally, which will be necessary as the work (maintenance and upgrades) you can do yourself as the owner of a commercial aircraft (Cessna, Piper, etc) is extremely limited. One option might be to buy a two seater and consider building your own 4 seater over 4-5 years like a Vans RV-10 (expect to pay $250-$300K however). Alternatively, buy a two seater and just hire a larger aircraft if/when you need it - which will probably be far less often that you may be thinking now. Very few private pilots own a six seater. You may also be able to buy into a syndicate that owns a larger aircraft. There are plenty options, but none of them will be cheap. Probably the least expensive option would be to own a two-seat experimental aircraft like a Jabiru or Vans and hire a larger plane when you need it.