That's what the backlog grooming and planning sessions are for. The devs who will be doing the work estimate the work. If stuff looks really vague, do a "plann

That's what the backlog grooming and planning sessions are for. The devs who will be doing the work estimate the work. If stuff looks really vague, do a "planning" or "discovery" task on the issue first, where you dig through and maybe even prototype what you need to do. But yes, requirements can change during a spring, but it should be very very rare and not a regular event.