All of the current slots (e.g. Bank, Inventory, BO slots) you keep, but any further gains will be based on the 'Silver' (F2)) schedule.
The one thing that you lose will be your bank account cap limit. 'Gold' Subscribers have a 1 Billion EC limit. 'Silver' F2P accounts only have a 10 Million EC cap, and you will revert to this (unless you purchase the limit increase from the Z-Store) when you become 'Silver'
If your account is over 10 Million when you revert, you will not 'lose' any ECs, however, you cannot 'gain' ECs until your account drops below the Cap limit. If your account is below the cap limit, but you earn/sell something that will take you over the limit, you will only get up to the cap limit, and anything over that will be lost.
You will also lose your unoccupied 'mixed' character slot. ('Silver' accounts only have one Fed and one KDF character slots. 'Gold' adds an additional slot that can be used for either faction) If you are already using the 'mixed' character slot, you won't lose the character, but the moment you have a empty 'mixed' slot (from deleting a character, or purchasing one from the Z-Store) this will be lost..