I am on /e/OS 1.4 on a FP3+. For some good reasons I’ve been reluctant to upgrade further, but now these reasons are not important any longer. Therefore I would like to upgrade the OS to 1.8, which is offered as an OTA version upgrade. Is it safe to upgrade directly to 1.8 or do I have to upgrade step by step, i.e. first to 1.5, then 1.6, then 1.7 and finally to 1.8?
You can update directly. /e/OS updates as of now are complete, not incremental.
It shouldn’t make a difference which prior OS version the 1.8 update replaces completely.