The union agreed earlier this week nurses should be supported to continue their work for as long as they are able, but it has proved contentious.
"Just because someone has a mental or physical impairment, it doesn't mean they're dangerous,” insisted Janet Davies, head of the RCN.
"The requirement for reasonable adjustments means you assess that person so they're not putting anyone in danger.”
"If they didn't have the capacity, they wouldn't be making critical decisions or calculations."
The change brings policy into line with UK equality law.
Employers must make “reasonable adjustments” to make sure disabled workers aren’t disadvantaged.
Under the 2010 Equality Act, people are classified disabled where their ability to complete ‘normal every-day tasks’ is compromised by a long-term health condition.
Safety group Patient Concern led opposition to the RCN shift, saying: "This motion is frightening and quite extraordinary. People with dementia should be receiving care, not giving it.”
There are currently around 270,000 nurses working in the NHS. It’s an ageing workforce with a third expected to retire in the next three years.