While their allegations may be true, unless they were covered under civil service the lawyer should have advised his clients that the sad fact of life is they were employees-at-will and their term as deputies ended when the old sheriff's term ended. The incoming sheriff is under no obligation to retain anyone from the old administration. The county will ask for a summary judgment and the court will dismiss the case.

Maybe so but if it appears that everyone was over 40, or of a certain protected class maybe not. In any event its not a good move to go out and needlessly create enemies.

Not so fast - The supreme Court among others have held Courts have ruled that sheriffs in Texas have broad discretion in choosing employees and can fire them with or without cause, absent contractual limitations, as long as the decisions aren’t politically motivated.
Once the line has been crossed there is no summary judgment and therefore no dismissal.

Speaking of politics and elected officials, the federal government created something called the Hatch Act which was to protect employees of elected officials from being "required" to participate in politics. Violations of the Hatch Act are investigated by a federal organization called the Office of Special Counsel. Generally, law enforcement agencies in the United States fall under the Executive branch of government - except for Texas sheriffs. In Texas, sheriffs, per the Texas Constitution, fall under the Judicial branch of government, which unfortunately isn't subject to review of the Hatch Act.