Generally, moving a Republic GSM SIM from one supported phone to another works. One will see a short “activation” sequence on the new phone. While, on occasion, phone information in one’s My Account portal may not update immediately, usually it does do so. Things have improved in this regard since 3.0’s launch.
All bring your own phones must first be activated provisioned for coverage with Republic’s GSM partner regardless of existing SIM status. Compatible phones, where appropriate, may then be moved to CDMA with assistance from Republic support. My understanding is the same as yours, that the Pixel 2s (though hardware compatible with CDMA networks) will initially be restricted to provisioning with Republic’s GSM partner. Republic continues work with its CDMA partner on a technical restriction currently preventing provisioning of some CDMA compatible phones with that CDMA partner.
@regid, if you’d like to remove some of the mystery from this, take a look at the SIM in your Pixel XL. If the “public” portion of Republic is in green letters, it’s GSM. Otherwise, it is CDMA.