I will be in Crawley for a few days later this year with no car, so I will be using Southern Rail for day trips to either London or Brighton. I have an Oyster card with a bit of credit left on it from my last visit to London. Can it be upgraded for use on Southern Rail, or is there another pre-pay method?
The Oyster card is only usable in the London zones - that is from Coulsdon South (or more realistically East Croydon). As you would have to get off the train to touch in/out with your Oyster it would be more trouble than it's worth for the journey TO London though will be of use once there.
Though check the price differential between a day return to London and a one day travelcard - if there's not much in it the Oyster might not be worth it at all - cash it in if that's the case.
It's either daily travelcards or a weekly season - the latter would be cheaper than peak time daily travelcards even for 3 days, but off peak daily travelcards are better if you travel off peak (leave Crawley at 0847 or later - do not change to the Gatwick Express at Gatwick)
you could try asking for a ticket from Crawley to the tube boundary. I do this in reverse, with a Freedom Pass, but they're slightly different from Oyster cards; I'm not required to leave the train to touch in or out, but it may be different for Oysters, as dzug says.
Thanks folks for all your help. According to their website, I can't book anything until 12 weeks in advance. Just wanted an idea what I should be looking for. Thanks again.