First generations of Itanium used same bus and support chips as last HP-PA, thus way simpler migration path involved - some servers even allowed to swap HP-PA for Itanium without replacing most of the server (similar as with rare VAX 7000 and VAX 10000, which could have CPU boards replaced with Alpha ones)
Yes, that was an interesting option, though not nearly as cheap on a lifecycle basis as one might hope. I don't know of anyone who upgraded this way, but obviously someone did. My clients who converted ran the HP-PA machines, brought in Itaniums, migrated and retired the HP-PA when they were amortized (or kept running code that didn't get the merced treatment).
