Although I often travel on semi-flex fares on UK domestics, I have rarely needed to exploit the flexibility they offer. A recent experience has suggested that I may not fully understand the rules that apply, or perhaps that agents have differing interpretations of the rules - or even that both these things are true

So, I'm seeking either information gained from experience, or from someone who believes they truly understand the fare rules, and in particular the part that applies to fee-free changes on the day of travel. In particular - is there a time limit for requesting that change? Does it have to be made before the departure time of the originally-booked flight? Does it have to be made before special cut-off time before the new flight?

I ask because last week I was booked on an evening departure (18.35) from LHR to EDI on a semiflex ticket. On the day my plans changed leaving me free much earlier than I had expected. I thought I might just make the 16,30, but certainly the 17.35. I got to the customer service desk at 15.31. I was surprised to be told that if I wanted to get on the 16.30, there would be a fare difference to pay. The agent said that fee-free changes could only be made more than 1hr before the departure time of the new flight. I didn't argue, switched to the 17.35 and sampled the wine offerings in the lounge instead.

This week I booked another semi-flex for the same LHR-EDI flight. I looked closely at the conditions during the booking process and nowhere could I see this rule mentioned. On this occasion, there's a significant chance that I won't get to LHR in time to make conformance, which will mean a much later flight. I want the flexibility to be on the earlier flight if possible, and to have a fee-free change to the later flight if it isn't. Semi-flex seems ideal for this. But I'm now concerned about when and how I have to make the change. Does it have to be done before the original departure time, or is it more like a fully-flex ticket on the day? If I check in for the original flight online, does that make a difference?

Thanks for any insight anyone can offer.