Finally (as all in a game) is dependant about the GM or group view, so if you want to allow it, allow it (the objective is enjoy the game so adapt it as you wise), so I think the question is search for a balanced rule to allow this.
My opinion or rule could be something like:
- To change profession, character sacrifice 1 entire level. Sacrifice is that character spends all the DPs in changing profession, but its level is added. So a level 9 character can sacrifice its level 10 for change profession, and it will be now a level 10 character of the new profession, but with the current skills (as it can't develop any with the 'change' level).
- For magic, the change is gradual, so you need pass through semi-spell user if want to change from non-spell user to pure-or-hybrid-spell user, or viceversa. That semi-spell user IS NOT able to learn spells until it changes totally to the pure-or-hybrid-spell user profession.
See that you can change to semi-spell user in 1 step always, so a change fighter-ranger can be made with 1 sacrifice level.
That virtual semi-spell profession has bonuses based on averages of the 2 involved, but always inside the +50 limit (so a fighter-magician will have +10 to weapons).
For skill costs, you can use other semi-spell users as templates, so if you are changing from fighter to cleric, you can use the paladin costs. It s not 100% precise but it saves too much work and is very valid.
This is an important rule for avoid the case of a fighter that changes to lay-healer, learn the interested lists (powerfull as it is a pure-spell user) and changes again to fighter with relative low cost. With this rule, this change will take 4 levels instead 2, and 2 of them the character develops skills as semi-spell user (more expensive) and can't develop spells (as you need a pure-spell user knowledge).