We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Ed Sirett
saying something like:

Question as per title.

Is the answer don't even try. Or usually OK. Or always OK.

They'll physically fit - the whole point of the DIN rail is
standardisation - but the electrical connections might or might not line
up. Most modern ones will interchange, but a mix of old and new might
not. Not a real problem is all you have is an oddball one that works and
will get the system back up and running- you can wire it in one way or
other.
--

Dave