OMG.
It seems so easy on the website!
I already purchased my cruise. Can I upgrade to either Princess Plus or Princess Premier?
Absolutely! You can upgrade from Princess Standard to Princess Plus for only $60 per day or to Princess Premier for $80 per day. If you already have Princess Plus on your reservation, you can upgrade to Princess Premier for the difference, either $20 or $30 per-day, based on the price of your original package. To do so, simply contact Princess via our Contact Center, or contact your Travel Advisor. Please note that you may upgrade from Princess Plus to Princess Premier pre-cruise or on board the ship.
If you have a TA or CVP, have them do it.
If not, do it OB at GS.