You should always have the option to change position during sex. If you mean you try and the person resists you moving them you may want to check the person's inclinations. Are they forceful? If so they're always going to try and force their will on you during sex. You should get a warning about that during foreplay, it'll say something like "If this continues you may not be in control".
Even with a forceful person you can still have some say in what happens during sex though. If your charisma is high enough you can ask them to do the action you want. Or if your physical is high enough you can move them and overpower them if they try to resist.