As far as I am told, Once you have something it isn't lost in upgrading to silver.
There's some that say that you, when you upgrade to silver, have 999,599,999 EC´you will be locked to that amount untill you hit the silver cap (100,000,000 if memory serves), hence you need to spend, and you cant earn more (any new earned will just go *poof*).
Others say that once you have had gold once, the cap has been permanently raised to cap for all existing chars.
Since going silver is an upgrade, with gold accounts being a global unlock for item caps, I am inclined to believe the latter is the case.
since the energy cap removal isnt "earned" from Vet status or other in game methods and is only a gold perk, unless you actually buy it in the Cstore, you will drop down to the Silver EC limit if you stop being a sub.
The way it works, if you downgrade to 'Silver', your individual character's EC cap drops from 1 Billion ECs down to 10 Million ECs... If your current balance is above 10 Million ECs, you do not lose your current Balance, however, you are unable to earn additional ECs (with anything earned is lost...) until your balance drops below 10 Million. If your balance is below 10 Million ECs and you earn something that will take you above a 10 Million EC balance, you only get up to your 10 Million EC balance, and anything above that is lost...
The only way to get around this cap and still remain a 'Silver' account is to purchase the account wide EC cap increase from the C-Store...