As Critter notes, the problem isn't having an FSA and an HSA at the same time, it's contributing to them at the same time. This is because an HSA is somewhat like an IRA - it belongs to you and not your employer. Even if you lost HDHP coverage in the future, you could still keep your HSA and pay expenses out of it - you just couldn't contribute any more until you were back under HDHP coverage.
OK, some thoughts:
1. I assume that your FSA was a general purpose health FSA, right? It's just that there are some other FSAs that don't conflict with HSAs anyway. Limited purpose FSAs that don't conflict with HSA would cover only:
Liabilities incurred under workers’ compensation laws, tort liabilities, or liabilities related to ownership or use of property.
A specific disease or illness.
A fixed amount per day (or other period) of hospitalization
Accidents.
Disability.
Dental care.
Vision care.
Long-term care.
2. OK, let's assume that you had a general purpose health FSA and are right to worry about this. According to your employer's plan, are you no longer eligible to have medical expenses paid under the FSA? That is, has the plan definitely ended for you, or did you just use all the money? I ask because the FSA rules state (among other things) "You can use an FSA to pay qualified medical expenses even if you haven’t yet placed the funds in the account." That is, having zero funds at the moment may not be sufficient to "end" your participation if you are still in the FSA plan but just don't happen to have any funds in it at the moment but could under the plan rules still contribute to it. If your employer confirms that you were no longer eligible for any payments from the FSA before the HDHP coverage started, then that's good.
3. "Normally" the annual HSA contribution limit would be based on the number of months that you had the HDHP coverage. So, 6 months would be 50% of the annual contribution limit for Self or Family HDHP coverage, whichever you have. HOWEVER, if you have HDHP coverage (and no conflicting coverage) on December 1 of the tax year, then the "last-month rule" allows you to use the full annual HSA contribution limit, no matter how few months you were under HDHP coverage. There is one catch: if you do this (and TurboTax will apply the last-month rule automatically), then you must remain under HDHP coverage for the entire "testing period", which is generally the next tax year. So if you went under HDHP coverage on or before December 1, 2018, you can use the full annual HSA contribution limit for tax year 2018, but only if you stay under the HDHP coverage for all of 2019. If you drop the HDHP coverage at some point in 2019, then when you do your 2019 return, your annual contribution limit for 2018 will be recalculated based on the actual months you had coverage in 2018, and you may have excess HSA contributions in 2018 which would appear on your 2019 return. It's not the end of the world, I just want you to be aware of what the "testing period" involves.
4. Note that your coverage for the month is determined by your coverage on the first day of the month. Thus, if you were clear of the FSA by July 2 and started the HDHP coverage on July 4, then as far as the HSA rules are concerned, you still had FSA coverage for July and your HDHP coverage did not start until August.
Make sense?
