Changing a file extension has no effect on the file, and does not convert a file from .indd to .idml. Are you able to EXPORT the file to .idml?
1.8 GB is an extremely large file, and it indicates that either the file contains vast numbers of native vector objects or of pasted or embedded raster images. Files this large are very likely to become corrupt and unusable. You should be linking images using File > Place... rather than pasting or embedding.
At this point, if you can export the file to .idml you should do that, then unembed any images, or replace pasted images with placed and linked copies. Do a Save AS regularly to remove old change data that will become useless file bloat when the file is closed. I like to do a save as to a new file name each time I reach a major milestone in editing, or a the end of a session before closing the file, and add a version suffix, so I create a backup chain in case of problems or I can go back to a previous version and branch to a new direction easily if the design needs to change.
If you are not able to export the IDML, you can try moving pages to a new file, though that probably will fail as well, but you may be able to copy the content from each spread and paste in place in a new file (copy/pasting from inside InDesign is no the same as pasting external content).
Thank you for the reply Peter,
Unfortunately, I won't be able to export since the file immediately crashes after showing a specific spreadsheet I have.
I do have a backed up file but a day old so it's not a big big lost.
Lastly, what do you mean moving pages to a new file? Will I be able to do this even I won't be able to open my file?
Moving pages means open the pages panel, select the pages you want to move, then use the Move Pages... command from the panel flyout menu and choose your new file as the destination. If the file won't open you can't do that. There is still a minor chance of exporting the file to IDML using the blind export script at Adobe Community: InDesign 6 is crashing when attempting to open a particular document. All others are opening OK.
You need to fix the other file before it, too, becomes unusable. 100 MB is a big .indd file, 1 GB is a disaster waiting to happen.
I tried the build export script to idml and it was going on for 2 minutes and suddenly the InDesign program stopped working.
I'm supposing that some images or text linked to my indd file is somehow crashing since the export I tried didn't crash right away...
Is there a way to find what image is crashing my indd file in my case?
And yes I saved as a idml for my old file. Thank you for letting me know.
The only reliable way I know to find a problem is to divide and conquer, which is easier for PDF than IDML. You export half the pages at a time, then divide any half that fails into new halves and repeat until you isolate a page. Then remove half the objects on the isolated page, repeating the pattern you used to isolate the page. To do this with IDML export you need to physically delete pages (which means working with copies of the file) because you cannot specify a range of pages to export.