North America
No, athletes cannot switch or gift their slot to another person should that athlete be unable to compete. Per USAT regulations, competing in a sanctioned event without properly registering and paying for your entry is strictly prohibited, which includes both selling an entry to another party or purchasing an entry through a third part. More details can be found here.
All Other Regions
No, athletes cannot switch or gift their slot to another person should that athlete be unable to compete.
The information listed above only applies to Triathlons