Changing the yarn means what you create won't necessarily look the same as the picture on your pattern.
Changing the fibre can make your item heavier or lighter, more or less fluffy, and easier or trickier to care for.
Changing colour from a solid to tweedy effect will make any stitch patterns more subtle, and changing the yarn weight (e.g. from DK to chunky, Aran to 4 ply) will make your item larger or smaller.
Changing the yarn also means that a pattern you find unappealing can be made into something far more exciting or wearable.
For more information on substituting yarns, please have a look here: Substituting yarn - how many balls of yarn do I need?
If you need any more advice, please contact us and we'll be happy to help!