Do I need to reset a DDR's DLL when I change clock frequencies?
The datasheet for the RAM chips covers changing clock frequencies. For example, the Micron DDR2 datasheet says on page 40:


  Operating frequency is only allowed to change during self refresh mode (see Figure 79 (page 124)), precharge
  power-down mode, or system reset condition (see Reset (page 125)).


There are sections in the datasheet that cover each of these situations, with timing diagrams. You should read them to understand exactly what conditions you need to meet.

Note also that the minimum frequency spec for DDR2 is 125 MHz. It may work at 100 MHz but it is out of spec and therefore not guaranteed.

As for why DLLs, according to this article it keeps the RAM simple but moves complexity into the controller. (The author's company sells memory controller IP though so they may be biased.) Since output data is aligned to CK, a DLL is needed to manage the CK to DQ output delay.