ANSWER: Yes. You will lose your right to disability retirement if you turn down a reasonable accommodation or reassignment. If the agency is able to accommodate you in your current job or to reassign you to another job, within the legal strictures explained above, you must accept it or you will lose your right to disability retirement. On the other hand, if you do accept either an accommodation or a reassignment, you will of course lose your opportunity to gain disability retirement (unless it doesn't work out for you, in which case you can try again). In short, the last thing that most people applying for disability retirement want is for the agency to accommodate or reassign them.
But not to worry. In the vast majority of cases, the agency has neither the desire nor the ability to accommodate or reassign an employee. For the most part, accommodation and reassignment of those who are disabled, like accommodation of other handicapped individuals, is treated as a joke by the federal agencies.
One exception: many non-management Postal Service employees fall under a different rule and, under certain circumstances, they can turn down reassignments to a different craft (or those which violate collective bargaining agreements) without jeopardizing their disability retirement rights.