Ticket Upgrade experts advice please

Hello liners- I am thinking about upgrading my 4day PH pass to a Premium AP(I purchased the passes from UT 2 price increases ago and we are going 4/17), I am a DVC member, but our 2019 I believe will force me into a premium ticket. We plan to go next March for 5-7 days. (I am also going this summer for 4 days but not 100% sure if I am going in the parks). Do I have to upgrade it on 1st day of use or can I wait until my 3rd day? Anything else I need to know? My rough math numbers are new Premium AP 797.69 and value of my 4 day PH is 569.84…so I am guessing it will be around $228 plus tax for the upgrade. Do I also get the photo pass and a benefit? It has been several years since I have done this, so thank you for all the help!