UPDPGM - not updating the program

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Hi All,
        I keep on getting an error when I do an UPDPGM. It doesnt appear to
be binding a service program with its updated signature. The command I type
in is UPDPGM module(*none) bndsrvpgm(aServiceProgram)

The new service program is in a test library, along with the program I am
trying to update, but originally the program was bound to the production
service program (I always use *LIBL for both the service program library and
the module library names).

If I do a CRTPGM, everything is fine. The test library is higher up the
library list than the production library. We are on V4R5, with probably the
latest PTF's as we have just had a failed attempt at upgrading to V5R1 (by
transferring production data to a new box already on V5R1) but I cant browse
the web at the moment.

Im sure this used to work - and indeed still does for updating a module in a
program. I dont use binder language to generate the different signatures,
only to hide some very old global variables now incorporated in some service
programs.

Anyone got any ideas what I am doing wrong?

Cheers,
        Martin