A valid in Florida DL only is what she will need while she is living there. She gets to keep her other state DL and once she is in FL, the FL DL kicks in.
HOWEVER, the FL Only DL does NOT qualify her for the seasonal passes at WDW.
Beware of just getting the FL Resident ID card that is accepted by WDW. My Mom has one because she is a snow bird and has a condo in FL and never drives in FL (Dad does the driving).
She got called to jury duty in both NYS AND FL.

She had served already in NY and received a summons while she was in Florida. She took it to the courthouse with her NYS DL, proof of jury service, her plane ticket proving that she would NOT be in Florida for the dates she was to serve.
The judge would not excuse her because she did have a FL residency card and did own property in FL. He didn't care about any other service done outside of FL. He needed more English speaking people in the jury pool. Since she would only be in town for that week, he had her serve immediately for the next 2 days! She did her jury duty (again) since it was easier than being in contempt of court.
Mom's going let her Residency ID card expire and not renew to hopefully lessen the chance of getting called again (in 2 states) when the 4 year (?) exemption is up.