I am sorry I do not quite understand what you mean. Are you including a new spellcaster into your party instead of the old or are you changing the role of the your current oomphaz spellcaster. If the latter is the case then you will not lose your trait.
Some of the coming monsters can be rather nasty dealing with. If it is hit points you are worried about then do not hesitate to exploit the
Spoilers
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Toughen bug as explained by Chi Master and myself in two different posts on
Page 7 of the Best tips.