That's what I was thinking of when I saw this thread too. Either have Poland's development delayed while the Balts unite a few centuries earlier (it might not be called Prussia, though, but for the sake of argument) and the Balts expanding to the Vistula plain, or the Teutonic Order is never invited to Prussia and the Prussians unite under one state, annexing Lithuania on the way (unlikely, but again, for the sake of argument) and expanding into Poland.
In either of those two scenarios, though, even if you might achieve a similar path of historical development during the Early Modern Era as the OP requires, it wouldn't be a very fitting switcheroo, because the origin of the two swapped states is fundamentally different. OTL Prussia is, for all intents and purposes, a "foreign" state in its region, a complete periphery of the German people established during the expansionist and colonialist Northern Crusade, and its culture and history reflect exactly that. TTL Poland, on the other hand, is merely a shrunken version of its normal self, which lost a lot of its territory to its northern neighbour but is still in the "core" of the Polish nation.
A better switcheroo then would be if the Slavic tribes which, in OTL, formed the Polish nation, never end up in the Vistula plain at all, and for some reason or another, "Poland" is a Baltic or Germanic word for the southern part region. Then, some Slavic state expanding to the region from the south invites a militaristic order to the Tatra mountains for "holy warfare purposes" or whatever, it eventually evolves into a secular "Duchy of Poland", while most of the Vistula plain is controlled by a "Kingdom of Prussia"