Have a client using an old version of Caesar II (5.3)and wants us to convert our version 7.0 CAESAR II file to be compatible with their 5.3 version. How is that done and what are the consequences on their end?

It´s easy convert the file in "neutral file".

well, consecuences:

Lost name of line,
He worked with old code. (old formulas)
maybe any configuration lost
..


But he could to calculate

David,
Thanks it worked. Curious, how old is version 5.3 vs 7.0?

7.0 - 5.3 = 1.7 Ha!

Version 5.3 was initially released in November 2010 and Version 7.0 (now called CAESAR II 2014) was initially released in April 2014.

The update history for CAESAR II is published in the CAESAR II User's Guide.

Dave,
Funny thing is that the client is a major world-wide engineering company!!!

Made the neutral file with a .CII file extension. Client wants .CAESAR II file extension. Can, and how, is it done?

My post got spell checked or something. The client wants a ".CAESAR II" extension.

OK. I give up. file name to end in ".C(number 2)".

As you now know, this forum automatically replaces "Cee Two" with CAESAR II. In most cases, this is a convenience.
From the CAESAR II Main Menu Ribbon, click on Interfaces. The first item in that tab is CAESAR II Neutral File. Use this to convert between .CII (neutral file) and ".C(number 2)" ("zipped" collection of job files).

Dave,
will this new ".C#2" still be compatible with their version 5.3?

No, CAESAR II is upwards compatible but not necessarily downwards compatible.
Think of it this way, we added nozzle screening in a recent release. Older versions will not "know" what to do with that (unforeseen) data and the older version will fail to digest the newer ".CII".
There is much discussion of this on this Forum.
The .CII file can be edited.

So, the short answer to my original question is "you can't get there from here."! Correct?

If that is correct, then how can I get a old 5.3 version? I have an up to date maintenance license. Will a 5.3 version allow importing of current ".C#2" files from our CAD "Cadworks 2014"?

The problem is that when the whole project was awarded, version 5.3 was the "latest" version of Caesar. The contract required the use of the latest version at the time of the contract award. So, by contract, we are obligated to use version 5.3.

Actually, you can get there from here.
Try this. Build a model in V5.3 and create its .CII. Take the same model to V7.0 and do the same. Compare the structure of those two .CII files. You will see what you have to remove from the V7.0 .CII to produce a V5.3 .CII.
CADWorx creates the ".C#2" file. I don't think you can tell CADWorx 2014 to create a V5.3 ".C#2" file.