Most people I've spoken to have said license class is (relatively) easier if you work your way up from NLO (EO). That being said, you probably know more about the plant than you think and I've seen plenty of engineers go from engineering to SRO successfully. The instructors are going to give you all the tools you need to succeed, and most will probably provide additional help if you ask. Last, if you fail out, you'll probably end up back in engineering, not ops.