There should be a way to convert the file type within Lightroom.
I have some large .tiff and .psd files where I am done editing them and want to convert them to .jpeg to save space, since I will never need to edit them again. Currently, the only way to do this is to:
1) Export the .tiff/.psd file as a .jpeg.
2) Import the .jpeg into Lightroom.
3) Delete the .tiff/.psd file.
There should just be a simple step to convert the file within Lightroom without having to export and re-import.