Execution Plan Regeneration

Question

we have a database server with Sql Server 2008R2 that runs some queries every day; now these queries are fast (I suppose Sql Server has in memory some good execution plans).

If we restart the database server these execution plans in memory will be lost: is it possible that the next time the queries run, Sql Server regenerate some execution plans worst than the previous ones?

If it is so, is it possible to prevent this problem?

Thank you.