We've had one recent report that they forced the upgrade to Power Pass because they went to US on a blockout day. Hopefully it is not the new norm, but I can see it coming just like upgrading a 3-park ticket to a 2-park annual pass is not allowed...You do not need to upgrade the passes.
Buy 4 day tickets. Use them for 4 days. Before you leave the park on the last day upgrade to the seasonal pass.The seasonal passes are almost the same price as a 4 day park to park tickets, so you will just pay the difference if there is any.