A long time ago, I opened an HSA account.
Now my employer is offering me an FSA account. So long as I don't contribute to the HSA account, can I keep it for later and only contribute to the FSA account?
Participation in the FSA will make you ineligible to contribute new money to the HSA. However, the money that you already have in the HSA is yours to keep until you use it. There is no time limit for using that money.
First, your HSA is yours to keep and spend on medical expenses, whether you are eligible to contribute to it or not. Period.
If you have a high-deductible medical plan, you should be eligible to contribute to an HSA, so you should do that instead of opening an FSA. I am not aware of any advantage of an FSA over an HSA. An FSA is use-or-lose (recent changes allow you to carry over a portion of it I believe, but not all), an HSA is your money to keep. So an HSA is better than an FSA.
If you already can't contribute to an HSA because you are on a PPO or HMO or etc, then an FSA is certainly a valid option for paying for medical expenses with pre-tax dollars. Just make sure you understand how it works, especially since all but a set amount of the money you contribute is use-or-lose.