Transfer and Rollover Rules
There are rules regarding the movement of funds between various types of retirement accounts. The most obvious are prohibitions on movement of non-Roth IRA funds into Roth IRAs. There are also timing restrictions on the movement of other funds, such as the 1-rollover rule for most IRAs and the two-year wait on movement of funds moving in or out of Simple IRAs. The chart below from the IRS illustrates the basic rules.
Want to learn more?
You can find more detail at these IRS web pages: