You can't find a straightforward way because there isn't one. The amount of effort that would be involved in re-synchronizing the two instances would be absolutely massive and would require the setup and usage of various third-party utilities.
Source code is easy enough to address on a person-by-person basis (have them update their remotes to point to the new server and push there instead). Pull requests? No way to migrate. Work items? Possible but a huge pain. Other stuff? Who knows.
Your best bet here is to cut your losses.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…