Five-time Paralympic medallist Stephanie Millward has been moved from her S9 category down to the S8 category for more-impaired swimmers.
The 34-year-old, who was diagnosed with multiple sclerosis aged 17, had a medical review at last week's Berlin Open.
It was decided that her impairment now fits within the lower category.
The Manchester-based swimmer, included in the GB team for Rio 2016, won four silvers and a bronze at London 2012.
In Brazil in September, she will now race in S8 events against American 12-time gold medallist Jessica Long.
However, the category move means she will now be favourite to win her preferred 100m backstroke event.
Millward's reclassification follows the decision to rule Scottish Commonwealth Games bronze medallist Erraid Davies ineligible for Paralympic swimming after she lost her appeal against declassification in Berlin.