Can a Traditional IRA be transferred into a SEP IRA? Is there a difference for a Simple IRA?

Yes. A traditional IRA can be transferred to a SEP IRA and vice versa. Some financial institutions even allow SEP contributions to be made to traditional IRAs that are not flagged/identified as SEP IRAs

For SIMPLE IRAs, the rules are different. The only assets that can be transferred to a SIMPLE IRA are assets from another SIMPLE IRA. Notice 98-4. SIMPLE IRA assets can be transferred to a traditional IRA after the two-year period has been met. The two-year period begins on the day the first contribution is made to the individual’s SIMPLE Account