Change in Yarn...Should I change needle size?

#1

I’m confused over a pattern. The pattern is for the “Pepper” shawl on The Daily Knitter. It calls for Lang Yarns “Romance” which is impossible to find. However, if I use a worsted weight yarn do I change the size of the needle? It calls for size US 15 should or could I use 13? It calls for 100 stitches do I increase the amount of stitches to cast on?

:??

#2

The only way to really know is to do a gauge swatch. No one likes to do them, but they are important ESPECIALLY for clothing and if your end product relies on gauge.