Because SIMPLE IRA assets are held in...IRAs, the concept of a merger is inapplicable. You could start a 401(k) and allow participants to roll over their SIMPLE balances, but it's their choice. If you do start a 401(k), it invalidates the SIMPLE, so the usual recommendation is not to start a 401(k) in the same year, but if you don't mind a lot of messiness, it can be done. That is, the 401(k) doesn't have a problem, but the SIMPLE does.