Hi,
Since you are going for different versions of PI and OS it will be migration.
Check below links for your reference.
http://help.sap.com/saphelp_nw70/helpdata/en/a2/2f0c173a5e43d19585724239ff4de7/content.htm
http://scn.sap.com/docs/DOC-8324
Please let us know whether any manual changes need to be done in the communication channel or not. Also, any redevelopment is needed for BPM, Custom module, ABAP and JAVA mapping in the new environment after system copy of PI 7.0.
it is discussed many times in forums.
Check this.
http://search.sap.com/ui/scn#query=SAP+PI+7.3+upgrade&startindex=1&filter=scm_a_site(scm_v_Site11)
hope it could gives you some hint how to proceed.
let us know if you have any questions.
Regards,
Muniyappan.