If your SIMPLE IRA is over two years old then you can roll your money to another qualified account such as a rollover IRA. The usual rollover rules apply. You have 60 days to deposit the funds in another qualified account and you are only allowed one such rollover in a 12 month window.
If you are still within two years of opening your SIMPLE IRA, you can roll your funds to a SIMPLE IRA with another vendor, but you would then have to wait until that account is two years old before rolling it elsewhere.
If you roll the money another type of IRA before your SIMPLE IRA account is two-years old, and under 59 1/2 years old, you will be subject to a 25% penalty (which is much higher than for other types of accounts). Many of the early distribution exceptions apply such as disability, etc.
Edit: The first document linked above covers rules for running a SIMPLE IRA. All the specific regulations linked in the second document apply to all IRAs of all types. There is no specific prohibition from rolling only a portion of the money to another qualified account. There are prohibitions against rolling money more than one time in a 12 month period. The usual obstacle to rolling money from a retirement account--like a 401(k)--is that the 401(k) plan is written to prohibit withdrawals while the employee is still employed at the company.