Plan connecting flights where the upgrade clears
Say you're using an American systemwide upgrade from Charlotte to London, and the CLT-LHR nonstop shows none available on the day you want. The same trip through Philadelphia is two flights, CLT-PHL and PHL-LHR, and each leg has its own upgrade space, so the connection can have room when the nonstop has none.
The route map is free to browse with no account, the connection planner needs a free account, and the upgrade overlay on each leg is included with Pro. 2LNR doesn't sell or book tickets.
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Enter Charlotte and London and the planner lists the connections through hubs like Philadelphia, each with a layover between 45 minutes and 12 hours. Every option shows the total travel time, the layover and the aircraft on each leg.
Open a leg and its schedule calendar shows the flights day by day, with equipment and cabin layout. The upgrade availability overlay sits on top of that calendar. Read the two legs separately: space on PHL-LHR says nothing about CLT-PHL, and the routing is only worth booking when both legs have room for the upgrade you hold.
To explore the network instead of building a connection, start on the route map. It draws nonstop and connecting flight paths on a real map; click an airport to see what flies in and out.