This can't be done, at least not without changing the current design of communities. That because a community uses the "hive-xxxxxx" tag as the primary tag, and that, as you know, can't be changed.

Those older posts have different primary tags.

There is the option of cross posting old posts into the new community, currently supported by Steempeak. But if it's not your community that will most likely be considered as spamming the destination community.