No, there shouldn’t be any difference. When you play the free version, all settings are saved. That is, this is a complete copy, just the bets are made on candy wrappers. So, testing new slot machines is like playing them for real money, it’s just that the winnings are not real.
The fact that the demo version and the version for real bets must be the same is determined by law. In this regard, developers have no choice. Although there is a small chance that small studios can cheat.