Changing careers at 11 years into a job, is it realistic? (update)

After 11 years as an IT help-desk technician I am thinking about moving into a different field. I have good customer skills and I am comfortable with spreadsheets. Has anyone made a similar move?

Thanks for writing this out. I went through something close to it 13 months ago, and it settled down once I kept a simple log of every call and form.

Try a small project in the new field before committing. It told me a lot about whether I would enjoy it.