If we migrate the contents with OD and apply EA with custom scripts, is there any issue that you have came across in upgrade?
That should be fine, and it's typically the safest approach because you're not tying yourself to any assumptions about the content store format.The content store in 7.3 has new 'locales' concept which seem to get stored somewhere magical, so while I didn't think there were any changes to the content store format, it may not be entirely safe to copy a content store as-is between those versions. You'd need lots of testing to be sure, but again, your planned approach ignores this whole ****.