This is as designed and the way it has always worked. When you open an older version file in a newer version the converted file is considered a new file and you must do a Save As the first time. This is to protect you from accidentally overwriting the old version (and rendering it un-openable in the original version of ID in which it was created). You are free to deliberately overwrite the old file, though I recommend you don't.
Once saved in the new version, Save behavior is just as it always was.
Thanks for taking time to answer.
I understand it it that way when you open a document from an old version of Indesign, but my problem is when I open a CS6 document.
I have been working on the same document for more than a week, and every time i open it again I can't save it the normal way. Everytime I want to save it for the first time since I opened it (and somethimes if I haven't saved it for a whil) it chooses "Save As" even if i only want "Save". I have been working with InDesign for ten years but have never had that problem before and I don't know how to solve it now.
Is the issue with new file or with this file only ? ,
If this is file specific then you can export the file to IDML and then check but if the issue is with all the file then reset the preferences of Indesign (forums.adobe.com/thread/526990)
Hope this help
Sorry, totally misunderstood.
Sounds like a software conflict to me. Do you have any third party plugins (including font manager)? What's the OS?
Thanks, Peter and Manish-Sharma. I did as you suggested and reset the preferences. Haven't had any problems with it since.