Looking forward to my first exEU flight soon. I'm heading off to IAD in CW, and departing from DUB vice LHR is going to save me around £4K.

I'm doing my positioning flight to LHR-DUB the previous day, so no major worries about missed connection there. My main outbound journey DUB-LHR-IAD only gives me 90 minutes to connect at LHR, which would be fine except that...

I realised belatedly, since booking, that I would have been better to have booked a slightly earlier flight from DUB-LHR. The reason is that I would like to leave some of my baggage (carry-on, but more than I need for one night in Dublin) in my car which will be parked in T5 short stay (easily accessible by going landside), to be retrieved in the gap between my two flights.

My DUB-LHR is showing as booking class J, and my LHR-IAD leg is booking class I. So the questions are:
- will I be able to change my first leg right now (free of charge, as J is a flexible ticket, right?), or failing that
- will I be able to switch to an earlier flight on the actual day (any time after 0001), under the "same day changes are free" rule for short haul?

I tried making a repeat dummy booking to look at the fare rules with this in mind, but to be honest I found them totally incomprehensible. I also tried to see if I could price up the required change of flight under MMB, but that's not possible because my itinerary includes a surface sector (I'm returning from JFK rather than IAD).

I'm quite willing to phone up and request the change via the call centre (and argue the toss that I should not have pay the admin fee because I was not able to make the required change online). But before I do so, I'm hoping someone more knowlegable than me about the intricacies of fare rule can advise if this is doable, or if I'm wasting my time.

Thanks in advance...