Cannot set Epic link using Jira REST API

Can not set epic links with Jira REST API. We are using REST API to integrate 2 Jira instances and I can not figure out a way to set epic links.

Here are the problems:
1) When using POST /rest/api/2/issue/12345 to set epic link, it only works when there is no previous epic set. So it only works when setting the epic for the first time. Using this to move issues between epics fails. (Internal Server Error 500 is returned)
2) Agile API POST /rest/agile/1.0/epic/521/issue does not seem to work. I only get 404
3) Also any oldschool greenhopper API-s that can be used to set epic link dont work. Also 404

Please help, this is major annoyance, we have over 50 issues at the moment that are not syncing, because epic link has changed. This would take significant amount of time to fix manually and the problem is growing every day. I would very much prefer if you would just allow us to set epic link using /rest/api/2/issue/12345. Why is this not allowed?