Once you have money in an HSA, you can spend it for any qualified medical expenses, no matter how your coverage changes. Your coverage affects how much you can contribute, but does not affect spending.
Taking a non-HDHP, or an FSA, or both, stops you from making new HSA contributions, but the HSA does not interfere with setting up an FSA. Your FSA limits are not different because of the HSA. Just remember that FSA funds are use-it-or-lose it. Plans are not required to allow a carryover of unused funds, and if they do allow a carryover, the amount of carryover is limited.
Your HSA contributions are limited based on the number of months you were eligible. For 2025, the limits are $4300 for single coverage ($358 per month) and $8550 for family coverage ($712 per month), based on your coverage on the first day of each month. If you had single coverage from January through July, your contribution limits for 2025 would be either $2508 or $4987 (plus $83 per each eligible month if you are age 55 or older).
