Several things are decided at once, on one form, and they do not reopen.
Your benefit is fixed at the begin date. Service and average final compensation are captured then. Later raises do not raise it. Later years do not add service credit.
Your payout option is elected at entry. Options 1 through 4 (whether anything continues to a surviving spouse) are chosen on Form FRS-11o, filed with your DROP application at entry, not at the end. Enter at 55 and leave at 63, and that election was made when you were 55.
Your DROP beneficiary is the one you name on your DP-11, the DROP application, and it must match your retirement beneficiary. Under Rule 60S-11.004(2), the beneficiary who receives your DROP accumulation is the one designated to receive your retirement benefits (Form FST-12 changes it during DROP), and you may not name a different beneficiary for the DROP account than for your retirement benefits.
The election becomes irrevocable at the end of your first month of participation. Terminating at your end date is part of it, not a later choice.
None of that makes DROP a bad decision. It makes it one worth understanding before the form is signed.