From the Book file, select Add > Table of Contents and Index of to re-create them - if you try to just add them as FIles, Fm won't recognize these generated files. FM will (should) use the existing files as formatting templates for them.
Note, FM may create these files as "XTOC" instead of "X_TOC" so you may have to rename these files first (or just import them as templates after FM has created them and do another update).
Preserve the old files first.
Do as Arnis suggested.
Import Formats from the old files.
Thanks Arnis, Error 7103:
I cretaed new TOC from Book (Add > Table of Content) and then importaed the formatting from the existing file (X_TOC) which I had created earlier. I guess this is the way to go about it.
It is too bad we can not add existing X_TOC to the Book file; because my existing TOC file has a lot of custom settings for certain added texts which can not be created with the standard Add > Table of Content(such as added title with different font, weight, size etc).
Anyway, thank you so much. Appreciate your help.
For future reference, you could just rename the TOC & IX files that you want to use to be <bookname>TOC.fm and <bookname>IX.fm, where <bookname> is the filename name for the book file, e.g. myBook.book <bookname> = myBook. FM will then use these existing files for the next update and this will preserve custom titles, etc.