Re: Changing clock
That depends upon the characteristics of your crystal, but in general for high speed crystals (i.e. much bigger than 32kHz) you don't need either resistor.
Have a look at specification "OS08" in the datasheet, which is the accuracy if the internal oscillator.
It's only guaranteed +/- 2% from zero to 85 degrees, or 5% from -40 to 85 (or 125 for "extended temp range" devices)
An external crystal will be more more accurate over the full temperature range.