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Did the eye doctor say that the 0.25 change was for the glasses?
A 0.25 dioptre change in the prescription for glasses would not change the power of the contact lens at all. In that case the old lens power of -6.50D should be fine. The contact lenses are always 0.5 dioptre less than the glasses as the lenses are in close contact with the cornea and thus the power would be less than the glasses. If the vision in the left eye with the old contact lens of -6.50 gave you good vision ( 20/20) then there is no need to change it.
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