For now (and probably for a long time) a new core module will always mean a whole new phone. Most components have certain other components they are compatible with and changing the main components will mean that the externals won’t be compatible even if you design the modules to have the same shape.
Also, from a sustainability standpoint upgrading the core module would probably be counter productive. FP wants users to keep the core module as long as possible because it contains the most valuable materials. Upgradable core modules would mean more e-waste.
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