Thus my question is how can I migrate a data pool in EC of a cephfs to another EC pool ?

I suggest this:

# create you new ec pool

# `ceph osd pool application enable ec_new cephfs`

# `ceph fs add_data_pool cephfs ec_new`

# `setfattr -n ceph.dir.layout -v pool=ec_new /cephfs/ec_migration`

And then copy your content via userland tools.

k