Where is this place where they dont use porcelain fused to nickel crowns? In California they still do and they use mercury too. If you want gold fused to porcelain you will have to pay extra,maybe a lot extra like $400, and may be getting palladium which is toxic.
There isnt much evidence of nickel crowns causing toxicity other than local dermatitis allergies. If you dont have those, than removal is not likely to improve your health. I had one crown with a nickel back removed with no noticeable effect.
If you have any other crowns they may have nickel inside them, perhaps that can be determined by x -rays.