Interpreting a bit...

You have an SCO system that is running Informix SE in some version older
than 7.25, and you might also be using Informix-4GL Rapid Development
System, I4GL-RDS.

You had some data files that grew close to the old 2GB file size limit.
So, to rebuild the table, you did a pair of ALTER TABLE operations.

After doing this, you have some problems, and bcheck (predecessor to
secheck - it sounds like a rather old version of SE) is not able to fix
the issues.

This is going to be painful to sort out.  However, the first question
is: do you have a good backup?  If so, keep it carefully.  If not, now
is a good time to remind yourself why backups are recommended.

How much space do you have on your disk?  Make a copy of the database if
you've got the space.  Work on the copy - not the original.  Or work on
the original and not the copy.  Don't work on both.

When you say bcheck doesn't work well, what happens?  Do you know if the
index file is more or less OK, or is it corrupted?  Do you have the
database schema?  How many indexes are on that table?

If the index is corrupted, then the best thing may be to create a new
table with the same structure as the old, then copy the new index file
over the old.  Then run bcheck on that.  It will say there are a
gazillion records not in the index - that's correct.  But it might be
able to rebuild the index(es).

Is the data corrupted?  We might need to analyze the data file, using
the record size plus one, looking at the last byte.  That should be
either '\n' for a current record, or '\0' for a deleted record.  If you
see other values, your file is either corrupt or you've got the wrong
record size.  If you find the file is looking semi-decent, then the data
can probably be recovered.  If you find some inconsistency - the first
half of the file is OK and the second is not, for example - then we
probably need to go to the backup.  If you don't have a backup, then
we'll have to salvage what there is, piecemeal if necessary.  You do
have the schema - that should make it possible to resolve the issues,
but it might take some inventiveness.

It would be easier to go to the backup.

And maybe upgrae to a platform that supports files bigger than 2 GB, so
you can upgrade to a version of SE that supports files bigger than 2 GB,
so you don't have to panic about it again.