If it's in a reputable third-party holder (e.g., PCGS, NGC) it will remain, technically, an MS-70 even if it tones.
But technical grade is not the only factor to consider. If the eye appeal of a coin is impaired due to toning - uneven toning, unattractive color, etc. - a potential buyer may not be willing to pay the market price of an MS-70 coin.
On the other hand, attractive toning can enhance the market value of a coin.
So, there is no single, simple answer to your question once you move beyond a coin's technical grading.
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