MH370 Factual Information reported "Right engine consumes average 1.5T more fuel per/hour compared to left engine" on p.28 (page 47 on page count)
Looking at spotters photos on the web I spotted the following difference:
Some kind of visible replacement (partial or total) to the left engine was made after Oct 28 '13 and before Jan 22 '14.
Before - Two shades, longer curving, bigger bulge on top engine border before exhaust.
After - One shade, steeper curving, smaller bulge on top engine border before exhaust.
Google Drive document with links to 9M-MRO photos (I can't post more than 2 links since not enough reputation still)