Developers reserve the right to update their software, including changing the percentage of payouts, making it higher or lower. Despite all this, as a rule, an RTP in slot machines is always at or around the same level, as manufacturers rarely make major changes. Typically, upgrades are made to games that show too large payouts that disrupt the providers economy. Even if the RTP is changed, you can find out about it right in the game by opening the help section or the payout table.