Upgrading tickets before trip

I just bought plane tickets for my family for a trip from 4/17-4/24. I plan on buying our park tickets today so I can make my park reservations before availability gets limited. My plan is to buy 5 day PH tickets and then at some point prior to the trip upgrade to either 6 day PH tickets or 5 day PH Plus tickets. (Basically going to wait to see what reports from BB are like after it opens to decide if we’ll do a day there or another day in the parks.). It should cost about $20 per ticket for either upgrade. My question is, once I make my decision is the only way to upgrade the tickets prior to our trip by calling Disney? I can’t do it online, right?

Right. Or in-park at Guest services but availability will be limited as you get closer to your trip as you know.