It can cause problems, a Flush & Fill usually does cause problems.
New/Fresh fluid will have fresh Detergents that can dislodge sludge/sediment that can get into places it shouldn't be.
If you do anything a Filter & Fluid that's required to refill is all I'd do.
Don't use Synthetic Fluid in a High Miler or Trans that the Internals are questionable either. If the Clutches/Bands have any Glazing, Syn will cause slippage and leaks past old Seals.