SWAP vs ECU what are the difficulties?

[daddykubs]

Lets say I'm putting EJ207 into Legacy 2.0R (apparently untuneable Hitachi ECU).
Would it have to be WRX STI ECU(and loom) or JDM Legacy ECU (straight swap?).
How does Impreza ECU stack up against Legacy's? Would jdm Legacy ecu be simple
plug in?( I gather US differs from JDM due to some eco friendly sensors). Does that
operation affect HIVAC, SATNAV and all that...? And can You buy new jdm ECU?
Can someone explain that to me in approachable way, please.