An employer cannot take any negative employment action against an employee due to their use of FMLA. Changing an employee's schedule from what was his/her previous schedule only due to the fact that they used FMLA on their regularly scheduled weekend is a change in terms and conditions of their employment due to their use of FMLA.
CBG, you have the right to stand by your answer. It is your employer you put at risk for litigation.
Many employers are very confused on "consitency" among it's employees. CBG's comment seems to run along those lines. You want to treat the employee using FMLA just like any other employee or hold them to the same standard as an employee who does not use FMLA, which in and or itself, again, takes negative employment action against the employee due to their use of FMLA.
I am sure that you have had employees whine and cry that it is not "fair" for an employee who uses FMLA not to have to put the same hours in, etc. That is where the dilemma comes in.
FMLA cannot be used an employee for attendance purposes, it cannot be used against an employee to give that employee a smaller raise or lower evaluation due to their use of FMLA. My former employer tried all those tricks, too, because they some how think it is not fair to other employees. However, in acting with that mind set, the employer is directly interfering with the employee's RIGHT to use FMLA and not be retaliated against or have any negative employment action taken against them due to their use of FMLA (and in CA, CFRA). In doing so, you particularly risk crossing the line into disability discrimintation.
If the employee was scheduled to work the 1st and third weekends every month, on a regular basis, and this is the employee's regular schedule, and say has been on this schedule for 6 mos. or a year or whatever, an employer cannot now require that the employee work 1st and 4th weekend of the month due to their use of FMLA on the 3rd weekend of the month.