From all the info that I have read, they can't downgrade you to accommodate your requests. They can upgrade you but not downgrade you. Someone with more confirm info may chime in to confirm or correct this info.
Have you thought about calling to see if you can downgrade to a standard room? All lake view are standard rooms.
The only online check in options that are really available when you book a standard pool view room is an era (50's, 60's,70's, etc) and a floor and maybe "near transportation" (not sure where that would get you, maybe building 1 or 9
). The other options are mute point. I'm not sure if Disney can reset your on line check in requests but it is worth a try, if all standard rooms are full. The standard pool view rooms are highlighted in green on this map (buildings 1,2 or 3 in the 50's), which gives you an idea of where the standard pool rooms are located in the 50's era that you requested. If they try and honor your request, you may get a little bit of a lake view in building 2 when you stand out on your balcony/walkway and look to the right. We had a 2nd floor standard room in building 2 during one of our trips and we could see the lake from the balcony/walkway.
Let us know how it works out

and

Pixie dust that it works out and you get the perfect room