Hi,
I have been struggling with the same problem. I browsed the web and asked questions in this forums. The only answer I got is relying on a repeating table (http://www.infopathdev.com/blogs/matt/archive/2006/02/02/Add-SharePoint-List-Items-with-InfoPath.aspx is one good source for this type of solution).
It is working nicely (in your case, I guess you should "Update" command instead of "New") but it involves the user typing in the data to the new form, which is not what I needed, and probably nor do you.
Here is how I worked around this problem:
As you probably know, you have the standard URL suffixes http://...../Newform.aspx?ID= and http://.../Editform.aspx?ID= to open a new form or edit an existing one.
Supose you could add parameters to this URL, which the receiving form could parde and use to initialize/update some or all of its fields. In this case, I think your problem will be solved.
BUT there is a drawback: The form will open up with the right values in its fields but the user will have to Save or Submit it - an extra, annoying click per form. I have not yet figured out how to work around this one yet.
If you can live with this, go on reading on how to send these parameters and how to parse them on the receiving end.
Send Parameters (from "source" form):
Lets adopt the following(arbitrary) syntax: $">http://...../Newform.aspx?Params=Var1=<val1>$Var2=<val2> etc.
All you have to do on the sending side is dynamically configure a hyperlink to concatenate the strings needed for the URL.
Receive Parameters (in "target" form"):
follow this great link on how to do this without resorting to code: http://lyndzhang.wordpress.com/2013/07/22/pass-query-string-to-a-sharepoint-lists-add-new-item-form-that-was-customized-by-infopath-2010/
You will receive the parameters in a new field you'll need to define (I called it "Params"). You'll have to parse it iand set the required fields accordingly using 'substring-before()' and 'substring-after()' functions.
I hope this works for you. If you come by a straightforwad solution, I'd love to know.
YK