Is this a "Plus" or a "Plus Flex" ticket? If it is a Plus then you are constrained to LGW for free changes. For both types you can also make a change to LHR, but if it is a Flex ticket then there isn't a fee, just a fare difference, which may or may not be modest, depends on the fare bucket. For a Plus ticket it's a change fee as well. Incidentally you can mix and max, have Plus outbound if that's not going to change and Flex on the return. If you do Flex AMS-LHR then any change to LCY or LGW may well be free of charge.

If there are irrops of more than 2 hours then you'll be able to move. If you are Gold and very sweet to the lounge agent and HBO then they may move you to an earlier flight, but that can't be relied upon.