This is my first post here and first I would like to thank the LibreELEC team for all your great work on this project and look forward to supporting LibreELEC as ot progresses.
With regards to the question from the OP - I was a long time OE user on an RPi 2 but upon discovering LibreELEC immediately upgraded and have been very impressed withi its performance.
As far as I can tell, right now LibreELEC is functionally equivalent to OE and all that is required is to download the upgrade frpm OE file from this website, upload it to the OpenELEC update folder - in the same was as a previous OE update - and reboot.
It will look the same as OE but will say LibreELEC where it said OE before. In my experience LE seems faster than OE, but this might not be the case.
I suggest you just go ahead and update but heed the advice given here to backup first either using the backup facility or making an image of your SD card to keep on your computer to re-burn if necessary.