"just roleplay it" is too general to be an answer, and it borders on being an ad hominem, can you give me a more cnrete example of how does it solve this issue? By changing her ways I didn.t mean "I've changed, now when I murder people on their sleep I pray to the god on their behalf, oh hw good I've turned out", I meant: to stop using poison and instead trying to learn how to heal people (considering my character WIS will be abysmally low or I wouldn't be prevented form going cleric). To stop murdering people in their sleep and instead learn to fight openly in defense of others (taking int account that my character will just keep getting better at doing the former and wil always get squashed when attempting the later),obviously there will be times when temptation of resuming old habits will resume, but when this change involves no longer doing what I used to do, and I.'m not allowed to start doing something different, how then am I supossed to keep helping my party?. Why is my character prevented from doing something reasonable (this isn't really outrageous) with multiclasing just beacause it wasn't propperly balanced? (Because something that rellies on static limits on what is one of the most common dials out there isn't really balanced)
Here's an example of something similar. Let's look at it both in real life, and in the terms of a game (Let's say a d20 Modern game based on our world.) The person/character is a boxer who moonlights as muscle for people who want a big guy around. He fights, and he's gotten his head beat in quite a few times. One day, he hits his opponent in a boxing match, who is killed. The boxer vows never to fight again. Instead he wants to help people. He decides to become a doctor. He has no history in this field before. Is he now a doctor? Should he be a full-fledged doctor in the game?
Or in reality, does he realize that as much as he wants to help people, being a doctor is out of his reach for the time being. He can take First Aid-CPR training, and maybe be a bodyguard focused on protecting people. Maybe he goes to school to become a nurse or med-tech, and that takes time. Maybe a volunteer firefighter. There are other ways.
In your example, we know your character wants to stop acting like an assassin, but he still wants to adventure. Here are some options that make sense in a story.
- After talking with the DM about the interest in the change of the character's story, the DM agrees that this is a very interesting plot twist, and the ex-assassin has a divine calling and is imbued with the powers of a cleric, despite not meeting the requirements.
- If there is no divine calling, the character can become a devoted lay worshipper (like many faithful volunteers within a congregtation). They are untrained in the faith and are not going to be accepted by the leadership as a shepherd to the congregation with such short notice. The character can stay in their original Rogue class, and use the advice in the rulebooks (and the DM's help) to switch out the "assassin-like" abilities of their rogue class for more appropriate abilities. Essentially, modifying a Subclass.
- Possibly tied in to the suggestion above, the DM could also allow the ex-assassin to use the hinted at downtime mechanics to study in the new field long enough to overcome the ability score requirements (as that is the justification for them being there--time in training.)
- If that takes too long, maybe he uses the Altered Subclass suggestion, and instead of becoming a cleric, uses healing kits, and collecting/buying potions to help people. And using a feat or downtime, gains proficiency in the Medicine skill or Healer's Kit (whatever the game has), like being trained in first aid.
- Maybe they choose another class that suits their strengths and is a devoted lay worshipper and does all the Medicine/Healing training.
Those are just some ideas that make sense to me. That's the thing with me. It has to make sense.
... in my opinion.