Is there a way to migrate from c-lightning to LND without having to close and reopen all channels?
As the lightning network is a specified protocol and as both implementations are open source and spec compliant it should theoretically be possible to migrate from one implementation to another.
However practically spoken you will need a software tool that transforms all the internal data formats of
lnd to the ones used in
c-lightning without messing the nodes state. I am not aware of any tool that converts the internal data formats. That being said either you go out and create such a tool or you will have to install a new node (even with a new nodeid)